Most people have heard of a so called "Safeword" before. In kink and BDSM, it plays a fundamental role to ensure the safety of every party participating.
A safeword is basically a code word which gets prearranged. If the safeword is used during the scene, the action will be stopped immediately. It can be used if a boundary is crossed or something is physically wrong. Safewords should also be included in vanilla sex.
A safeword doesn't necessarily have to be verbal. A gesture or similar can be used as a safeword as well. Especially if speaking may not be possible in a situation.
Using "Stop" as a safeword is just fine. However, to some people, saying "Stop" during a sexual activity could also mean "Oh yes, please keep going". Using an abstract which isn't usually used during sex (ex. "Pineapple") may be a better option.
An improved variation of using only one safeword is the traffic light method. Red, orange (or yellow) and green.
Red: Stop immediately! Something is seriously wrong.
Orange/Yellow: I'm not quite comfortable with the situation. Please be more careful/change the action/ask me what's going on.
Green: Everything is okay. Keep going. (Usually used when the other party asks you about your wellbeing and you want to tell them that everything is fine).