I've noticed that besides metric units and AU's there is another unit used to measure distance in Eve - lightyear. So far I know it's related to use of jump capable ships but I don't how much distance is that; for example how many systems can you reach within 1 ly range?
Matt [ Editor ]
Light year is the measure of distance between systems.
It is useful in Two circumstances, both when using non gate means of travel.
Jump Drive capable ships
Bridging ships via (Soverinty Jump bridges, Titans or Black Ops)
Each system has a map position and the grid has spatial properties. It is possible if you look at the map in 3D mode to guage distances.
Best way to tell exact distances is to use a database tool like Dotlan's Jump Planner or EVEHQ's Map tool.
Seem to remember somewhere that the average adjacent solar system is approximately 2.3 l/y apart. Don't know if that's correct or not but there we are.
In all cases the distance in Light years is a function of Racial Isotope consumed by the jump bridge/drive etc. The other factors are Ship mass for Bridges (and ship type as genearlisation for Jump capable ships), and Jump fuel Conservation skill level.
EDIT: and due to being in a meeting and slow to type, Serker has pretty much beaten me too it. +1 him.
As pointed out, Light years also effects the remote sensing for PI, although I always suspect that it operates more like supply chain management than pure light years sphere of influence... it does state categorically however distance is in Light years.