1. Literally, to grab or hold onto something intensely or with a lot of strength. He had a panic attack halfway up the ladder due to his fear of heights and seized upon the rungs for dear life. The child seized upon her father's arm during the scary parts of the film.

2. By extension, to accept, adopt, or undertake something with great enthusiasm or zeal. The boss decided seized upon my idea of developing a smartphone app to accompany our newest product. You need to seize upon opportunities like these before they pass you by.

3. To resort to some tactic or plan, as out of desperation or necessity. Jake seizes upon any reason he can think of to get out of doing his chores. You can't just seize upon any old excuse and expect that to fly in this office.