Definition of tip:
(n) :
in the sense of gratuity, originally a small amount of beverage (i.e.
tipple) left in a glass for the bartender; a small
amount of money left for a
bartender,
waiter,
taxi driver or other servant as a
token of appreciation
(v) :
to inform police or investigators of a potential clue; to inform a trader of inside information regarding a company's performance