A light-year is the distance that light travels in a year.
The count of light-years to some object under
observation also represents how far into the past this view of the
object represents, i.e., if a star is ten light-years away, then when
we see it, our view is of that star's appearance ten years ago.
In the science of astronomy/astrophysics, the unit light-year is
often used for communications to the public, but the unit parsec (pc)
is generally used in scientific calculations and communication.
A light-year is approximately:
The International Astronomical Union (IAU) has standardized a precise definition of the light-year:
the speed of light in a vacuum (c) times a Julian year, 365.25 days of
standard (SI) seconds. This distance is an exact number of
meters because the standard meter is defined in relation to c and
the standard second, such that a light-year is exactly
9,460,730,472,580,800 meters.