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early 13c., "act performed in accordance with prescribed usage," especially a religious or ceremonial one, from Old French observance, osservance "observance, discipline," or directly from Latin observantia "act of keeping customs, attention, respect, regard, reverence," from observantem (nominative observans), present participle of observare (see observe). Observance is the attending to and carrying out of a duty or rule. Observation is watching, noticing.