Both an official of a political party in the legislature who enforces party discipline, and the written direction that official issues to members on how to vote.
The link between the three-line whip and disqualification under the Tenth Schedule is a common cross-topic polity question.
The whip is unconstitutional in the sense of being absent from the constitutional text, but it is recognised by convention; only defiance of a whip on the issues covered by the Tenth Schedule risks disqualification, not every internal whip.
Party enforcer and its written voting order; three-line whip defiance can trigger anti-defection disqualification.