to perceive or take note of (something), esp by concentrated attention
to act in due conformity with (e.g. a law)
They were careful to observe local customs
to celebrate or perform (e.g. a ceremony or festival) according to a prescribed or traditional form
to utter (something) as a comment
They observed that things weren't what they used to be
to make a medical or scientific observation on or of (somebody or something)
observable adj
[Middle English observen via French from Latin observare to guard, watch, observe, from ob- + servare to keep]