Skin fold

Skin folds or skinfolds are areas of skin where it folds. Many skin folds are distinct, heritable anatomical features, and may be used for identification of animal species, while others are non-specific and may be produced either by individual development of an organism or by arbitrary application of force to skin, either by the actions of the muscles of the body or by external force, e.g., gravity. Anatomical folds can also be found in other structures and tissues besides the skin, such as the ileocecal fold beneath the terminal ileum of the cecum.

Skin folds are of interest for cosmetology, since often they are undesirable features, and for medicine, because many of them are susceptible to inflammation and infection.

https://en.wikipedia.org/wiki/Skin_fold