A coordinate used on the celestial sphere just as longitude is used on Earth. An object's right ascension is measured in hours and minutes eastward from the vernal equinox.
The axis of rotation of an Equatorial telescope mount aligned with the earth's axis of rotation. It's calibrated in in 24 hour segments, since the Earth takes 24 hours to complete one rotation.
the arc on the celestial equator extending eastward from the vernal equinox to the meridian of the object in question in the equatorial coordinate system. This coordinate corresponds to longitude on the Earth. The other coordinate in this system is called "declination."