In GEANT tracking a particle through a geometry of objects consists in calculating a set of points in a seven-dimensional space (x, y, z, t, , , ) which is called the trajectory of the particle. This is achieved by integrating the equations of motion over successive steps from one trajectory point to the next and applying corrections to account for the presence of matter.
To have a detailed description of the kinematic of the particle it would be necessary to calculate a trajectory point every time the component of the momentum change. This is not possible because it would mean calculating an enormously large number of points. Processes like the deviation of a charged particle in a magnetic field, the loss of energy due to bremsstrahlung and ionisation or the deviation due to elastic electromagnetic scatterings are essentially continuous. An arbitrary distinction is thus made between discrete and continuous processes, controlled by a set of thresholds which can be set by the user. A particle trajectory is thus a set of points at which a discrete process has happened.
The tracking package contains a subprogram which performs the tracking for all particles in the current event and for the secondary products which they generate, plus some tools for storing the space point coordinates computed along the corresponding trajectories.