Forming adjectives and some nouns.
[Latin -aeus, -eus or Greek -aios, -eios, plus -an.]
The ending most often appears in adjectives derived from proper names and place names. The ending is no longer productive, new forms being created in -ian instead.
Many derive from names associated with the classical periods via their Greek or Latin adjectival forms. For example, Euclidean, of the system of geometry based on the work of the Greek mathematician Euclid, comes from the Greek adjectival form Euklideios. Others are Herculean, from Hercules, via Latin herculeus, and Pythagorean, from Pythagoras. Some, derived from proper names of classical times but now usually written with lower case initial, are cyclopean (from the one-eyed giant Cyclops), protean (from the Greek sea god Proteus who could change shape at will), and terpsichorean (from the Greek muse of lyric poetry and dance, Terpsichore).
Some adjectives have been formed from place names that derive from Latin or Greek: European (Latin europaeus, based on Greek Eurōpē, Europe) and Mediterranean (Latin mediterraneus, from medius, middle, plus terra, land).
A few adjectives in -ean do not come from proper names: subterranean, existing, occurring, or done under the earth's surface, comes from Greek subterraneus, below the earth; hyperborean, of an inhabitant of the extreme north, is from late Latin hyperboreanus, from Greek huperboreos, ‘beyond the north wind’.
A few adjectives relating to places have been created in English: Andean, Antipodean, Caribbean (also a noun), Hebridean, Ecuadorean, Tyrolean. Some other adjectives derived from personal or place names actually contain -an added to a word ending in e or ea: Boolean, Carlylean, Eritrean, Kampuchean, Shakespearean, Zimbabwean.
See also -acea (for -acean).