1.
reaction; response; feedback
A visible or audible response to something, especially an expressive or animated reaction. In Japanese, particularly associated with exaggerated physical responses seen in comedy and variety TV shows.
From English "reaction," but in Japanese usage, リアクション often specifically refers to animated, visible, or physical responses — especially the kind of exaggerated reactions prized in Japanese comedy and variety shows. Comedians known for physical reactions are called リアクション芸人.
The native Japanese word 反応 covers the broader meaning of "reaction" in both scientific and everyday contexts.