-ics

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Science, art, study, or knowledge of, or skill in: photonics.
2. n. Actions, activities, or practices of: athletics.
3. n. Qualities or operations of: mechanics.

Century Dictionary and Cyclopedia

1. n. A termination of Greek origin, denoting a science or an art. Words with this termination are properly plural, but are now commonly regarded as singular, being often accompanied by forms actually in the singular, as mathematics, hydrostatics, esthetics or esthetic, metrics or metric, etc. In some cases the singular alone is in use, as in logic, music, the adjective being then exclusively in -ic-al, as logical, musical, while in a few a distinction of meaning has grown up, as between physic and physics. Any adjective in -ic, applicable to a branch of knowledge, may have an accompanying noun in -ics.

Wiktionary

1. n. Forms nouns referring to fields of knowledge or practice.
2. n. Forms nouns relating to activities

GNU Webster's 1913

1. A suffix used in forming the names of certain sciences, systems, etc., as acoustics, mathematics, dynamics, statistics, politics, athletics.