Periapsis: The point in its orbit where a satellite is at the least distance from the Earth.
Apoapsis : The point in its orbit where a satellite is at the greatest distance from the Earth.
Prograde: The direction in which a ship is traveling along its orbital path. Since orbits are elliptical, it is always tangent to the orbit at the point where the ship is.
Retrograde: The reverse of Prograde, backwards along the orbital path.
Radial : The radial direction is the direction inside the orbit, towards the focus of the orbit. i.e towards the center of the Earth.
Anti-radial: The Anti-Radial direction is the direction outside the orbit, away from the focus. i.e away from the center of the Earth.
Neglecting the presence of an atmosphere. What is the most efficient (least fuel consuming way) to lower one's orbit and perform a reentry.