noun
passing acquaintance; slight acquaintance
Used to describe knowing someone only by sight or having met them just once. Often appears in negative constructions like 一面識もない (don't know someone at all).
彼とは一面識もない。
I don't know him at all.
一面識の相手に道を尋ねられた。
A stranger asked me for directions.
Compound of 一面 (one meeting/one side) and 識 (acquaintance/knowledge). The exact historical derivation is uncertain, but the meaning is transparent from its components.