Wiktionary
prep.phr. (context idiomatic English) In the process of doing something considered criminal or otherwise wrong; (non-gloss definition: used to emphasize the eye-witness evidence).
Usage examples of "in the act".
B replies by wireless telegraphy that he is now in the act of bombarding the chief manufacturing city of A by means of three raider airships.
There was courage in the act, as it developed when a smoking fuse came flying out of the door like a thin gray snake.
It may or may not have worked, but if it has we've caught someone, right in the act.
But they are two different worlds, Fred-two completely different worlds-and he didn't really see it happen, never actually caught either one in the act of going or coming.
Its rotation, however, had so positioned the apparatus that he collided with the central unit, which faintly resembled a wide hand cupped as in the act of scratching.
He made as if to wipe his face, and in the act settled the mote just beside his eyelid.
Either side could try to sneak its men across the river without being caught in the act by the other.
Her sensual pleasure in the act was obvious, but it did not distract her from her thoughts.
If there is no truth in the act of contrition, there is no merit in confession, and to absolve her would have been deadly sin.