Analogues of alkenes in which a doubly bonded carbon atom is replaced by a heteroatom, e.g. $\ce{H2Si=CH2}$ methylidenesilane (silene less preferred), $\ce{MeN=CH2}$ N-methylmethanimine.
Source: PAC, 1995, 67, 1307. (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1340 [Terms] [Paper]