The description of the word you requested from the astronomical dictionary is given below.
'light‧year, plural lightyears
A lightyear is the distance that light travels in a vacuum in a year. So, a lightyear is not a measure of time but of distance. A lightyear is not an official unit and it is therefore not specified which definition of year you should use to calculate its length. You may use the length of the Julian year. With that year, a lightyear corresponds to about 9,460,730,000,000 km or 5,878,640,000,000 miles or 9.46073 Pm or 9.46073 × 1015 m or 63241.08 AU or 0.3066 pc.