1 tr. (often foll. by that, how + clause) perceive, note; take notice of; become conscious of.
2 tr. watch carefully.
3 tr. a follow or adhere to (a law, command, method, principle, etc.). b keep or adhere to (an appointed time). c maintain (silence). d duly perform (a rite). e celebrate (an anniversary).
4 tr. examine and note (phenomena) without the aid of experiment.
5 tr. (often foll. by that + clause) say, esp. by way of comment.
6 intr. (foll. by on) make a remark or remarks about.
Derivatives:
observable adj. observably adv.
Etymology: ME f. OF observer f. L observare watch (as OB-, servare keep)
Useful english dictionary. 2012.