Analogues of alkenes in which a doubly bonded carbon atom is replaced by a heteroatom, e.g.
H2Si=CH2 methylidenesilane (silene less preferred),
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 (https://doi.org/10.1351/pac199567081307)