What is "matheme"

The matheme (from "lesson") is a concept introduced in the work of the 20th century French psychoanalyst Jacques Lacan. The term matheme 'occurred for the first time in the lecture Lacan delivered on November 4, 1971...Between 1972 and 1973 he gave several definitions of it, passing from the use of the singular to the use of the plural and back again'.