Secondary is an adjective meaning "second" or "second hand". It may refer to:

• Secondary, in American football and Canadian football, the group of (usually at least four) defensive backs
• Secondary, in geosciences, an obsolete name for the Mesozoic (see also Quaternary)
• Secondary circuit, electrical circuit in a Transformer in which current is produced by electromagnetic induction, as opposed to a primary circuit
• Secondary color, as opposed to primary color
• Secondary consumer, in an ecology
• Secondary dominant, in music
• Secondary education
• Secondary functional group (or usually ion), in organic chemistry, attached to carbon atom connected to 2 other Carbon atoms
• Secondary market, an aftermarket where financial assets are traded
• Secondary neoplasm
• Secondaries, the second-largest group of remiges (wing feathers), which attach to the inner lower arm
• Private equity secondary, an investment in an existing private equity fund or other alternative asset