棄権【きけん】
abstention (from voting), renunciation (of a right), withdrawal (from a contest)
放棄【ほうき】
abandonment, renouncement, renunciation, resignation, abdication, surrender, relinquishing, waiving, giving up
棄却【ききゃく】
rejection, dismissal, turning down, abandoning, renunciation
破棄【はき】
tearing up and discarding (e.g. documents), destruction, annulment, cancellation, abrogation, voiding, breaking (e.g. a treaty), reversal (of an original ruling on appeal), quashing