An Environmental Effect is a special type of effect caused by terrain that you are passing through. For example, Tar, found in Ascalon, which lowers your speed by 33% or Icy Ground, as found in Marhan's Grotto, which causes you to move 70% slower.
Environmental effects have a green border like enchantments, but cannot be removed.
For a list of all environmental effects, see Category:Environmental Effects.