-oid is a suffix much used in the sciences and mathematics to indicate a "similarity, not necessarily exact, to something else". According to the Oxford English Dictionary, -oid is derived from the Latin suffix -oides taken from Greek and meaning "having the likeness of".

Thus, asteroid means "like a star" and rhomboid means "like a rhombus". There are many examples of such words: anthropoid, alkaloid, factoid, humanoid, planetoid, trapezoid, android, and so forth.

When nouns formed using -oid are turned into adjectives, the suffix usually becomes -oidal.

New Oxford American Dictionary has:

-oid: suffix forming adjectives and nouns:

1. Zoology denoting an animal belonging to a higher taxon with a name ending in -oidea: : hominoid | percoid.
2. denoting form or resemblance : asteroid | rhomboid.

ORIGIN from modern Latin -oides, from Greek -oeidēs; related to eidos ‘form.’

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