-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 lozenge, or rhombus". There are many examples of such words: anthropoid, alkaloid, factoid, trapezoid, and so forth.

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