Term
Momentum
The motion still carrying your body after a volley. If it takes you into the kitchen, it is a fault.
The motion that is still carrying your body after you hit the ball (Section 2, Definitions). The momentum from a volley lasts until you get your balance back or stop moving toward the non-volley zone (kitchen) (Section 2, Definitions). If this momentum takes you into the kitchen or makes you touch the non-volley zone line (kitchen line) after a volley, it is a fault (Section 2, Definitions, 3.A.4.c, 11.A.1, 11.A.2).
This is true even after the ball is dead (9.B.1, 11.A.2). For details, see Entering the kitchen from your volley's momentum is a fault.
See it in a figure
This figure is from the rule "Entering the kitchen from your volley's momentum is a fault".
How to read the sources: numbers like (8.F.4) are rulebook sections, and links are outside sources. An "Author's note" is the author's personal experience, not an official rule.