Forming adjectives, and some nouns.
[French -atique or Latin -aticus, often based on Greek -atikos.]
Some of the many adjectives in this ending are dramatic (via late Latin from Greek dramatikos); rheumatic (Old French reumatique, or via Latin from Greek rheumatikos); pneumatic (French pneumatique or Latin pneumaticus, from Greek pneumatikos, from pneuma, wind). The ending also appears in adjectives formed from English nouns: operatic, acrobatic, lymphatic.
Some adjectives in -atic have become nouns as well, through a shorthand in which—for example—aquatic plant has been abbreviated to an aquatic, or in geology, an erratic boulder became an erratic, one thought to have been brought from a distance by glacial action. A few are as often nouns as they are adjectives: lunatic, fanatic.
For adjectives in -cratic, see -cracy.