An astringent is a substance which causes biological tissue to contract or draw together. There are a number of uses for astringents medically, and many cosmetic companies also sell astringents for skin care. The term is also used to refer to tart foods which cause the mouth to pucker, such as lemons, pomegranates, and persimmons. Tannins, such as those found in teas and wines, are also astringents, since they cause the mouth to feel dry and constricted. Many of these tannins, such as oak bark, are used to produce astringent products for both medical and cosmetic use.

The word's origins can be found in the Latin astringere, which means “to bind fast.” An astringent can be said to have “astringency” when someone is describing its properties. When an astringent is applied to living tissue, it does indeed cause the tissue to bind fast to itself, causing it to shrink. This property can be extremely useful for a variety of applications.

In internal medicine, astringents are used to shrink mucus membranes. By reducing swelling, a doctor can identify areas of bleeding or irritation more readily. The use of a medical astringent can also act to reduce unwanted discharge, which will make patients more comfortable. Many doctors also prescribe astringents to relieve skin irritations such as those cause by fungal infection and insect bites. Calamine lotion is an example of a topical medical astringent designed to reduce irritation, as is witch hazel. Pet guardians may have used stronger astringents, in the form of styptic pencils or sticks, which are designed to be applied to nails which have been cut too short in order to stop the bleeding.

In cosmetics, astringents are used to firm and tone the skin, constricting the pores and creating a protective layer of firm tissue between the under layers of skin and the elements. A cosmetic astringent is sometimes called a toner, and it is usually applied after bathing but before the application of moisturizers. Aftershaves are also astringents. Using an astringent after shaving can help to reduce the pain of minor cuts while also soothing the skin.

Applying excessive astringents, especially harsh ones such as alcohol, can be drying. This is why many cosmetic astringents are followed by moisturizer, to keep the skin moist as well as firm. There is also some debate in the medical community over whether or not people with acne should use astringents. Some people argue that the astringents may have a positive impact, while others fear that astringents restrict the pores, leading to an increased likelihood of blockage and infection.