An arcsecond (arcsec or just second or sec) is a unit of angle or arc, commonly used in astronomy for celestial sphere coordinates and distances across the sky. A degree (unit of angle corresponding to 1/360 of a full circle) is divided into 60 arcminutes (arcmin or just minute or min), which, in turn, is divided into 60 arcseconds, making an arcsecond 1/3600 of a degree or 1/1,296,000 of a full circle, or 2π/1,296,000 of a radian. A milli-arcsecond and micro-arcsecond are 1/1,000 and 1/1,000,000 of this value.
Right ascension is commonly measured in hours (which might logically be called angular hours), 1/24 of a full circle, corresponding to 15 degrees. Fractions of an hour are also expressed as minutes (1/60 hour) and seconds (1/60 minute), which are not the same angular size as the arcsecond and arcminute associated with degrees. (As far as I can tell, the terms arcsecond and arcminute are unambiguous, but one must be careful with the terms second and minute.)
Astronomers sometimes avoid hours, minutes and/or seconds: radians may be used, or hours for right ascension may be avoided in favor of degrees, or degrees (and/or hours) may be expressed in decimal fashion when specifying more angular precision.