belonging to the second string of a team / substitute or replacement / second most important

(physics) A wave of fluctuating temperature or entropy propagated in the superfluid phase of liquid helium

The upper joint of the leg of a fowl, especially when used as food

(mathematics, logic) Denoting the second in a numerical sequence of models, languages, relationships, forms of logical discourse etc. / Of secondary importance.

(biochemistry) Any substance used to transmit a signal within a cell, especially one which triggers a cascade of events by activating cellular components

The second-lowest rank in the Boy Scouts of America, between tenderfoot and first class.

(neutral or mildly positive connotation) Of a standard or grade one level below the best. / (negative connotation) Inferior in quality or standing.

A person who plays second string. A person who is kept on a sports team as a backup in case a "first-string" player is unavailable.

The second gearing available from a gearbox. / (idiomatic, figuratively) Mediocre, sluggish performance.

An angle, one sixtieth (1/60th) of a minute of arc or one 3600th of a degree.