A headmate is an "Alter" of someone who has Dissociative Identity Disorder. A headmate is their own person who has split off from the original person's mind, who has their own separate wants, goals, needs, gender, age, etc.

These headmates are usually formed as a coping mechanism, helping the person with DID to survive the traumatic experiences that they have been a victim to. A collection of headmates including the host is called a Multiple/Median System.

Due to the large amount of systems on Tumblr, it has been wrongly labelled as a "Tumblr trend" and been accused of mocking mental illness, despite it being a mental disorder in itself.