Thrust is a reaction force described quantitatively by Newton's Second Law when a system expels or accelerates mass in one direction to propel a vehicle in the opposite direction.
Total force is equal and opposite to the mass m times the acceleration a experienced by that mass:
1. An aircraft generates forward thrust when the spinning propellers blow air to the back of the aircraft. The forward thrust is proportional to the (mass of the air) multiplied by (average velocity of the airstream).
2. Similarly, a ship generates forward thrust (or reverse thrust) when the propellers are turned to accelerate water backwards (or forwards) .... the resulting thrust pushes the ship in the equal and opposite direction to the sum of the momentum change in the water flowing through the propeller.
3. A rocket (and all mass attached to it) is propelled forward by a thrust force equal and opposite of the momentum change experienced by the exhaust mass accelerating out from the combustion chamber through the rocket nozzle.