1. To take action in a situation. The report is due tomorrow, so I need you to act on it immediately. Please act upon this subpoena for records at once.
2. To take action that is strongly influenced by certain information or advice. ("Upon" is most commonly used to convey this meaning.) Acting upon my doctor's orders, I cleared my schedule and spent the week recuperating at home. A; "I can't believe he said that about us!" B: "Well, he's probably just acting on his mother's instruction, and that woman hates us."
3. To impact in some way. The barking dogs next door are acting on my patience.
See also: act
act (up)on something
1. to take action on a particular problem. (Upon is more formal and less commonly used than on.) I will act on this immediately.
2. to take action because of some special information. The police refused to act upon his complaint because he was an ex-convict.
3. to perform on something, usually the stage (in a theater). Ken has never acted on the stage or in front of a camera.
1. Also, act upon. Conduct oneself in accordance with or as a result of information or another action, as in I will act on my lawyer's advice, or The manager refused to act upon the hotel guest's complaints. [c. 1800]
2. Influence or affect, as in The baby's fussing acted on the sitter's nerves. [c. 1800]