Thrust is the force exerted by one body A on another body B, in the direction perpendicular to the surface on that body B. If the force is exerted at angle to the surface, then the component normal to the surface is the thrust.
The thrust is the reaction force in the Newton's second and third laws.
When a rocket throws out hot burning gases from its exhaust, the reaction force exerted by the gases is called the thrust. Due to the thrust the jet or rocket moves forward. It is equal to the rate of change of momentum of the gases from the burning of fuel.
Its units are Newtons.