Definitions of past:

• noun:   a verb tense that expresses actions or states in the past
• noun:   the time that has elapsed

  Example: "Forget the past"

• noun:   a earlier period in someone's life (especially one that they have reason to keep secret)

  Example: "Reporters dug into the candidate's past"

• adjective:   earlier than the present time; no longer current

  Example: "Time past"

• adjective:   a verb tense or other construction referring to events or states that existed at some previous time

  Example: "Past participle"

• adverb:   so as to pass a given point

  Example: "Every hour a train goes past"