The Collaborative International Dictionary of English v.0.48:

Observation \Ob`ser*va"tion\, n. [L. observatio: cf. F.
   observation.]
   1. The act or the faculty of observing or taking notice; the
      act of seeing, or of fixing the mind upon, anything.
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            My observation, which very seldom lies. --Shak.
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   2. The result of an act, or of acts, of observing; view;
      reflection; conclusion; judgment.
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            In matters of human prudence, we shall find the
            greatest advantage in making wise observations on
            our conduct.                          --I. Watts.
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   3. Hence: An expression of an opinion or judgment upon what
      one has observed; a remark. "That's a foolish
      observation." --Shak.
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            To observations which ourselves we make
            We grow more partial for the observer's sake.
                                                  --Pope.
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   4. Performance of what is prescribed; adherence in practice;
      observance. [Obs.]
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            We are to procure dispensation or leave to omit the
            observation of it in such circumstances. --Jer.
                                                  Taylor.
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   5. (Science)
      (a) The act of recognizing and noting some fact or
          occurrence in nature, as an aurora, a corona, or the
          structure of an animal.
      (b) Specifically, the act of measuring, with suitable
          instruments, some magnitude, as the time of an
          occultation, with a clock; the right ascension of a
          star, with a transit instrument and clock; the sun's
          altitude, or the distance of the moon from a star,
          with a sextant; the temperature, with a thermometer,
          etc.
      (c) The information so acquired; as, to record one's
          observations carefully.
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   Note: When a phenomenon is scrutinized as it occurs in
         nature, the act is termed an observation. When the
         conditions under which the phenomenon occurs are
         artificial, or arranged beforehand by the observer, the
         process is called an experiment. Experiment includes
         observation.
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   To take an observation (Naut.), to ascertain the altitude
      of a heavenly body, with a view to fixing a vessel's
      position at sea.
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   Syn: Observance; notice; attention; remark; comment; note.
        See Observance.
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WordNet (r) 3.0 (2006):

observation
    n 1: the act of making and recording a measurement
    2: the act of observing; taking a patient look [syn:
       observation, observance, watching]
    3: a remark expressing careful consideration [syn:
       observation, reflection, reflexion]
    4: facts learned by observing; "he reported his observations to
       the mayor"
    5: the act of noticing or paying attention; "he escaped the
       notice of the police" [syn: notice, observation,
       observance]

Moby Thesaurus II by Grady Ward, 1.0:

165 Moby Thesaurus words for "observation":
   Parthian shot, accordance, acquittal, acquittance, address,
   adherence, advertence, advertency, affirmation, alertness,
   allegation, announcement, answer, apostrophe, apprehension,
   assertion, assiduity, assiduousness, assumption, attention,
   attention span, attentiveness, attitude, averment, awareness,
   bugging, care, carrying out, climate of opinion,
   cloak-and-dagger work, cognizance, comment, commentary,
   common belief, community sentiment, compliance, conceit,
   concentration, concept, conception, conclusion, conformance,
   conformity, consciousness, consensus gentium, consideration,
   counterespionage, counterintelligence, crack, criticism,
   declaration, dictum, diligence, discharge, discovery, ear,
   earnestness, electronic surveillance, espial, espionage, estimate,
   estimation, ethos, examination, exclamation, execution, expression,
   eye, fancy, feeling, following, fulfillment, general belief,
   greeting, heed, heedfulness, heeding, idea, image, imago,
   impression, inspection, intellectual object, intelligence,
   intelligence work, intentiveness, intentness, interjection,
   judgment, keeping, lights, looking, lookout, mark, memory-trace,
   mental image, mental impression, mention, military intelligence,
   mind, mindfulness, mystique, note, notice, notion, obiter dictum,
   observance, opinion, perception, performance, personal judgment,
   phrase, point of view, popular belief, position, posture, practice,
   presumption, prevailing belief, proclamation, pronouncement,
   public belief, public opinion, question, reaction, recept,
   reflection, regard, regardfulness, remark, representation, respect,
   satisfaction, say, saying, scrutiny, secret police, secret service,
   sentence, sentiment, shadowing, sight, spying, stakeout, stance,
   statement, subjoinder, supposition, surveillance, survey, tailing,
   theory, thinking, thought, trailing, utterance, view, viewing,
   watch, watching, way of thinking, wiretap, wiretapping, witnessing,
   word