An old term, which is not recommended, for alkenes, especially those of low molecular weight.
An old term for alkanediyl groups commonly but not necessarily having the free valencies
on adjacent carbon atoms, e.g.
propylene (systematically called propane-1,2-diyl).
PAC, 1995, 67, 1307
(Glossary of class names of organic compounds and reactivity intermediates based on
structure (IUPAC Recommendations 1995))
on page 1314