A primary candidate's ability, as perceived by a political party's ideological base, of having the best chance of later winning in the general election.
Many Democratic primary voters voted for John Kerry in 2004 because they believed he was the only candidate with enough electability to be able to beat Republican George W. Bush in the Presidental Election.
The chance a political candidate has of actually getting elected as represented by the media and public opinion.
Smart Friend - "That Ron Paul guy is right on literally everything its not even funny!"
Dumb Friend - "Yeah I know, hopefully one of the candidates with electability starts stealing his talking points so that we can get some of those ideas implemented."
Smart Friend - "I'm moving to Iceland."