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Comedones are acne caused by sebum and dead skin cells blocking the opening of the hair follicle. If the opening is closed and appears white, it is called a closed comedone; if it is open and appears black, it is called an open comedone.

Papules are red, painful acne where the skin is raised in a small, firm, red bump, but no visible pus is present.

Pustules are red, painful acne where white or yellow pus is visible in the center of the red elevation.

Nodules are large, firm, and painful inflammatory acne that develops deep within the skin. Since the risk of scarring may increase, it is advisable to have a prompt medical evaluation.

Closed comedones are acne that appear as small white or skin-colored bumps, caused by sebum and dead skin cells accumulating inside while the follicle opening remains closed.

Open comedones are acne where the follicle opening is open and sebum and dead skin cells accumulate, making the surface appear black. This black color is different from accumulated dirt.

Inflammatory acne is a collective term for acne with prominent inflammation, such as red and painful papules, pustules, and nodules. Since the depth and pain levels vary, they are not treated as a single type.

Cystic acne is a common term for severe acne where inflammation grows deep in the skin, causing pain when touched and persisting for a long time. In actual medical evaluation, deep nodules and cystic changes are distinguished.

A hair follicle is a pocket inside the skin where hair grows. It is connected to a sebaceous gland, so if the opening becomes blocked or inflamed, acne or folliculitis may occur.

Sebum is an oily substance produced by the sebaceous glands that protects the skin surface. Changes in the amount and secretion process can affect the formation of comedones and inflammation.

A sebaceous gland is a skin structure connected to the hair follicle that produces sebum. If sebum and dead skin cells accumulate at the opening of the follicle, a comedone may form.

A pore refers to the opening on the skin surface where hair and sebum emerge. The size of the pore alone does not determine the severity of acne.

A microcomedone is an early stage of acne where sebum and dead skin cells have begun to accumulate at the opening of the hair follicle, but are not yet clearly visible to the eye.

Follicular keratinization is a process where keratin at the opening of the hair follicle does not shed on time and clumps with sebum to create a blockage. This is one of the critical processes in the formation of comedones in acne.

Acne bacteria are bacteria that can live in the skin's sebaceous glands and hair follicles, and they are involved in the inflammatory response within blocked follicles. The mere presence of the bacteria does not determine the severity of the acne.

Post-inflammatory erythema refers to flat red or pink marks that remain where acne inflammation has subsided. This is different from pitted scars.

Post-inflammatory hyperpigmentation refers to flat brown or grayish-brown marks that remain due to increased melanin after acne or irritation has subsided.

Atrophic acne scars occur when skin tissue does not recover sufficiently after inflammation, leaving the surface pitted. This is different from flat red or brown marks.

Icepick scars are atrophic acne scars that have a narrow opening and appear as sharp, deep indentations in the skin.

Boxcar scars are atrophic acne scars that are relatively wide with clearly defined borders.

Rolling scars are atrophic scars that appear as wide, gentle, wave-like depressions caused by adhesions beneath the skin.

Acne marks primarily refer to flat red or brown traces left where acne has subsided. These are different from scars where the skin is indented or raised.

A non-comedogenic label means the product is designed to reduce the likelihood of creating comedones that clog pores. It is not a guarantee that acne will not occur for everyone.

Topical retinoids are topical medications used to reduce the pore-clogging process and treat comedonal and inflammatory acne. Dryness and stinging may occur, so the application method may need to be adjusted.

Benzoyl peroxide is a topical ingredient used to reduce acne-related bacteria and inflammation. Dryness or stinging may occur, and it may bleach towels and fabrics.

Byeonjeung is a Korean medicine diagnostic process that determines the direction of herbal medicine prescriptions by examining how heat, sebum, dryness, and pain differ even for the same acne, and whether digestion, sleep, and menstrual status repeatedly change when acne worsens.

Damp-Heat is a Byeonjeung term used in Korean medicine treatment to describe the simultaneous appearance of red and pustular acne, oiliness, sticky sweat, and patterns of digestion and bowel movements.

The skin barrier refers to the function of the outer layer of the skin to retain moisture and prevent irritating substances from easily entering.

Irritant contact dermatitis is a type of dermatitis where irritants such as cleansers, exfoliants, or acne medications damage the skin barrier, causing redness, stinging, and flaking.

Purging is an informal term referring to a change where clogged comedones appear to surface more quickly at the start of certain treatments. Not all worsening can be explained as purging.

Exfoliation is a care process that physically or chemically reduces the dead skin cells on the skin surface. Overlapping multiple methods can worsen stinging and redness in acne-prone skin.

Oral acne antibiotics are medications prescribed for a certain period to reduce bacteria and inflammation in inflammatory acne.

Antibiotic resistance is a state where bacteria become less affected by a specific antibiotic. This is considered when determining the duration and combination of acne antibiotics.

Isotretinoin is an oral retinoid medication used for severe acne or cases that have not responded sufficiently to other treatments. Pregnancy-related safety guidelines must be followed, and any physical discomfort must be continuously monitored.

Maintenance therapy refers to continued treatment after acne has subsided to reduce the occurrence of new acne.

Acne extraction is a medical procedure to remove the contents of existing comedones or certain types of acne. It does not treat the entire process of new acne forming.

Recurrence refers to a state where acne that had decreased after treatment appears again. This does not mean that the previous treatment was meaningless.

Adult acne refers to acne that continues into adulthood or starts anew. This does not mean it only occurs on the jawline.

Worsening before menstruation refers to a repeating change where acne increases or becomes deeper around the same time before menstruation begins.

Androgens are a group of hormones present in both men and women that can affect the activity of sebaceous glands.

Polycystic Ovary Syndrome is a condition where changes related to ovulation, menstruation, and androgens can appear together. It is not diagnosed based on acne alone.

Medication while preparing for pregnancy is a process of reviewing the safety of oral medications, topical medications, Korean herbal medicine, and supplements starting from before pregnancy is confirmed.

Folliculitis is a condition where inflammation occurs in the hair follicles where hair grows. Even if it looks like acne, the cause and treatment may be different.

Malassezia folliculitis is a condition where yeast living on the skin causes inflammation in the hair follicles, which can result in itchy, similarly sized acne-like bumps on the chest, back, and forehead.

Rosacea is a chronic skin condition characterized by persistent redness and burning sensations in the center of the face, as well as vascular changes and breakouts.

Perioral dermatitis is a type of dermatitis characterized by clusters of small red papules and stinging sensations around the mouth, nose, and eyes.