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Be careful in spelling equivalent (and, by extension, equivalence and equivalency). (There appears to be a tendency for the '-i-' to infect the adjacent vowel to make equivilent, which is WRONG.

  • The word comes from the Latin root æqui- 'combining form' of the adjective æquus, which is the same as equal in English, and valens, the present participle of the verb valÄ“re 'to be worth'. So equivalent ~ 'of equal value'.

A similar pattern, where the idea of 'equality' is formed from equi-, can be seen in a number of English words (OED lists 137), of which many are combinations found in specialized areas of the physical sciences. The most common non-specialized such words currently include:

  • equilateral, 'having equal sides' (commonly used about 'triangles' in school geometry);
    • formerly, this was also expressed by equiangular, which is necessarily the same thing in plane geometry;
  • equilibrium, or 'equally balanced', most often used of forces in physics and mechanics, and of competing powers in politics and history;
  • equipoise, which is more a 'balance of weights' in the physical sciences, although it too is used figuratively in political sciences;
  • to equivocate is 'to use words ambiguously', 'to use words with double meanings'. Nowadays, it is mostly used in a pejorative sense to mean 'to prevaricate', "To mean one thing and express another" (Johnson's Dictionary, cited OED). It has several derivatives, like equivocation and equivocator.