Circular velocity:
The velocity an object must have parallel to the surface of another object in order to stay in a circular orbit around it.

Perigee:
The closest point to Earth for an object in orbit around Earth.

Apogee:
The farthest point from Earth for an object in orbit around Earth.

Ellipse:
A type of mathematical curve. The path of one object orbiting another.

Parabola:
A type of mathematical curve. The path an object launched from a planet, for example, would take if it had exactly enough velocity to leave forever the gravitational pull of the planet. This is called the escape velocity.

Escape velocity:
See parabola, above.

Hyperbola:
A type of mathematical curve. The path an object launched from a planet, for example, would take if it had more than enough velocity to leave forever the gravitational pull of the planet. This is called the hyperbolic velocity.