How to explain a trajectory and hits in G4?

a trajectory means a track?
a hit means mean a reaction of inelastic?

A trajectory is a list of coordinate locations corresponding to interactions of a track from its creation to its stopping.

A rich trajectory includes additional intermediate locations, chosen to better model the actual path a particle would take in the real world.

A track is the position, momentum, energy and identity of a particle traversing the geometry. The track is “instantaneous”, in that it carries only its current state (location, momentum, etc.), not its whole history.

A step is the transition from its current state to its next state. The step carries the position, momentum etc. of both the starting and ending point of the transition, along with information about energy deposited in the current material, secondary particles created, etc.

A hit is a record of the energy deposited during a step, including the location, and possibly information about the track’s kinematics at that point.