The description of the word you requested from the astronomical dictionary is given below.
The escape velocity is the lowest speed that you need to have to escape from the gravity of a celestial body without needing a rocket engine or other form of propulsion anymore, if there is no atmosphere or other source of friction to slow you down. Escaping here means that you can get as far away from the celestial body as you want. The escape velocity depends on your location. "The" escape velocity of a planet is the escape velocity that you'd need at the surface of that planet.