noun

• (philosophy) An abstract archetype of a given thing, compared to which real-life examples are seen as imperfect approximations; pure essence, as opposed to actual examples.
  "The idea that the same experiments always get the same results, no matter who performs them, is one of the cornerstones of science’s claim to objective truth. If a systematic campaign of replication does not lead to the same results, then either the original research is flawed (as the replicators claim) or the replications are (as many of the original researchers on priming contend). Either way, something is awry."
• (obsolete) The conception of someone or something as representing a perfect example; an ideal.
• (obsolete) The form or shape of something; a quintessential aspect or characteristic.
  "The remembrance whereof (which yet I beare deepely imprinted in my minde) representing me her visage and Idea so lively and so naturally, doth in some sort reconcile me unto her."
• (music) A musical theme or melodic subject.