Definitions of opposition:

• noun:   a body of people united in opposing something
• noun:   a political party opposed to the party in power and prepared to replace it if elected

  Example: "Her Majesty's loyal opposition"

• noun:   a contestant that you are matched against
• noun:   a direction opposite to another
• noun:   the relation between opposed entities
• noun:   the action of opposing something that you disapprove or disagree with

  Example: "Despite opposition from the newspapers he went ahead"

• noun:   the act of opposing groups confronting each other

  Example: "The invaders encountered stiff opposition"

• noun:   an armed adversary (especially a member of an opposing military force)