How do astronomers define a light year?

The problem with the definition of light year (ly) is the duration of year. IAU year is Julian 365.25 days of 86400 s/day. An improved ly is for Gregorian 365.2425 days

Light second (ls) is 299792458 meters, nearly, The ly is the distance traveled by light, in a sidereal year.

Older approximation to sidereal year is 365.25 days. Later approximation was 365.2425 days. For more significant digits (sd), it is 365.256363 days.

Anyway, it is safe to use the common (tallying) 5-sd approximation 1 ly = 1.0950 E+11 meters.

It is better to define day = 86400 s and express year in days.