Euler diagram a logic diagram invented by the mathematician Euler that represents standard form statements in syllogistic logic by two circles and a syllogism by three circles. In modern adaptations of Euler diagrams, distributed terms are represented by complete circles and undistributed terms by partial circles (circle segments or circles made with dotted lines): Euler diagrams are more perspicuous ways of showing validity and invalidity of syllogisms than Venn diagrams, but less useful as a mechanical test of validity since there may be several choices of ways to represent a syllogism in Euler diagrams, only one of which will show that the syllogism is invalid. See also SYLLOGISM , VENN DIAGRA. R.P.