Somebody ranking next below a commander. / Someone ranking next below the person in charge.

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

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

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

plural of second cousin

plural of parallax second

Alternative spelling of second-in-command

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

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