at high pressure—
1. (of matter) when strongly pressed:
- Steam and oxygen at high pressure are introduced at the lower end of the gasifier.
2. acting with great energy and speed:
- As for Robert, he, of course, was living at high pressure all round.
under high pressure—
1. = at high pressure 1:
- The new material loses its resistance only under high pressure.
2. when strongly persuaded or compelled:
- Working under high pressure for sustained periods results in stress.