(mathematics) discrete mathematics

a photograph of oneself, a photographic self-portrait / (especially) a selfie

to take a photograph of oneself, to take a selfie

talking about oneself (especially, when the audience is not interested)

soul-searching

one's own concern or problem

(idiomatic) selfishness, self-centeredness

head / counter for large animals

head (body part) / (anatomy, archaic, possibly obsolete) the fontanelle part of the skull

a head, as when counting people, or cattle or other livestock