noun

• A person who opens something.
• A device that opens something; specifically a tin-opener/can-opener, or a bottle opener.
• (in combination) An establishment that opens.
• (card games) The player who starts the betting.
• (card games, in the plural) Cards of sufficient value to enable a player to open the betting.
• (metalworking) A person employed to separate sheets of hot metal that become stuck together.
• (theater) The first act in a variety show or concert.
• (cricket) A batsman who normally plays in the first two positions of an innings.
• (colloquial) The first in a series of events, items etc.; the first remark or sentence of a conversation.
• (sports) The first game played in a competition.
• (sports) The first goal or point scored.
• (fishing) A period of time when it is legal to commercially fish.
• (baseball) A pitcher who specializes in getting the first outs of a game before being replaced, either by a long reliever or a pitcher who would normally start.