That which stands for something, a symbol, icon, glyph. In linguistics, the signifier is the word, which (except in the case of onomatopoeia) has no relation/an arbitrary relation to the signified.

Roland Barthes and others took this idea further than pure linguistics and posited that anything, anything, can be a signifier - can, combined with its culturally accreted associations and meanings, be a sign. So semiotics (a branch of theory that was named before it existed) grows.