pons asinorum

Etymology
From, from + genitive plural of. lit. 'bridge of donkeys'.

Noun

 * 1)  A method for finding the middle term of a syllogism in Aristotlean analytics.
 * 2) An obstacle which will defeat a beginner or foolish person.
 * 3)  A proposition in Euclid stating that the angles at the base of an isoceles triangle are equal.

Etymology
Literally “bridge of donkeys”, attributed to (14th century) but first attested in Petrus Tartaretus (d. 1522), who cites it as a common appellation for the device in syllogistic reasoning “because of its apparent difficulty”, i.e. it separates experts from “donkeys”. The application to Euclid appears to postdate its use in logic.

Noun

 * 1)  a method for finding the middle term in an Aristotelian syllogism, or a diagrammatic representation of this method
 * 2)  the geometric theorem  that the two angles opposite the equal sides of an isosceles triangle are also equal
 * 3)  a difficult early test that must be passed if further progress is to be made