1
general::
Phrase(s): in the act (of doing something)
while doing something. • There he was, in the act of opening the door. • I tripped while in the act of climbing.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
1. In the process of doing something. For example, The police caught the robber in the act, or I was in the act of closing the window. [Second half of 1600s] 2. Performing sexual intercourse, as in Her father caught them in the act. [Late 1500s]
American Heritage Idioms