anagen

This term is a specialized biological descriptor used primarily in dermatology and trichology. It describes the specific window of time when the hair follicle is actively synthesizing keratin to grow the hair shaft. Because it refers to a distinct physiological stage, it is used in a clinical or scientific register rather than in casual conversation.

As a technical term for a biological phase, it is treated as an uncountable noun. It describes a state or a period of activity rather than a discrete object that can be counted. You would refer to the anagen phase or the anagen period rather than referring to multiple anagens.