[Either French -ide, via Latin -idis, or Latin -ides, in both cases derived from Greek.]
This ending forms the names of some plants whose family name ends in -aceae (orchid, family Orchidaceae); of some animals whose taxonomic name ends in -idae (felid, a mammal of the cat family, Felidae; noctuid, a moth of the family Noctuidae); or of plants or animals in classes whose name ends in -ida (arachnid, a member of the class Arachnida).
It is also used to form the names of structural constituents in biology (chromatid, each of the two thread-like strands into which a chromosome divides during cell division; plasmid, a genetic structure in a cell that can replicate independently of the chromosomes).
In astronomy it can denote a meteor in a shower radiating from a specified constellation (Geminids, from Gemini; Leonids, from Leo); a cepheid is a variable star like the first one observed, Delta Cephei.
The ending can also indicate a member of a dynasty or family (Abbasid, a member of a dynasty of caliphs who ruled in Baghdad from 750 to 1258; Sassanid, a member of a dynasty that ruled Persia from the early 3rd century AD until the Arab Muslim conquest of 651).
See also -oid.