Slang: Five years in prison.
Another prison slang term that often crops up on television cop shows (e.g. Law & Order). In New York, you would be most likely to serve your nickel at someplace like Attica or Ossining, or so the writers would have us believe.
Unlike its cousin the bullet, the etymology of this term is pretty obvious: one nickel = five cents = five years.
Sources: www.convictsandcops.com, dictionary.prisonwall.org.