verb
(transitive) To notice or view, especially carefully or with attention to detail.
(transitive) To follow or obey the custom, practice, or rules (especially of a religion).
(transitive) To take note of and celebrate (a holiday or similar occurrence), to keep; to follow (a type of time or calendar reckoning).
(intransitive) To comment on something; to make an observation.
noun
(archaic) An observation (remark, comment or judgement).