Humans use gestures which is a movement of a part of a body in a certain pattern to express and idea or meaning. Gesture recognition is the science of computers applying algorithms to try to interpret human body language.
Gestures are in their simplicity just a set of movements that have been defined as a gesture. If data that has been registered about position, scale factors, velocity, rotation, angle and more fits the predefined logic a gesture is consider as executed.
We tend to differentiate between two main groups of gestures, offline and online gestures. This has nothing to do with internet access and are terms frequently associated with touch interaction, but these make sense with motion tracking as well.
Offline gestures are gestures that are processed after you as a user have interacted with the object
Online gestures are gestures that have a direct effect on the object
If you select a menu by a Keytap gesture and then scale the screen with a scaling gesture you have first performed an offline gesture then an online gesture.