Pons asinorum is a Latin phrase that literally means "asses' bridge", or perhaps "bridge of the asses". However, the term is generally not used literally. Rather, it is a way of referring to the fifth proposition in Book 1 of Euclid's Elements, the classic text of geometry. Apparently the idea is that understanding this proposition is a bridge over which asses will not venture. The term apparently dates back to medievel times (when a bridge for donkeys would perhaps have been a more common sight), with the earliest citation in English in the Oxford English Dictionary dating to 1751, from Smollet: "Peregrine...began to read Euclid...but he had scarce advanced beyond the Pons Asinorum, when his ardor abated."

This proposition states that if you construct a triangle with two sides the same length (an isoceles triangle), then the angles opposite those two sides will be equal. Perhaps the slickest proof of this proposition is the one attributed to Pappus (fl. 320 CE). To understand the proof, you must know that in the fourth proposition Euclid has already shown the if two trianges have two sides and the angle between them in common, then all of the corresponding parts of those trianges are the same. Consider a triangle whose vertices are called A, B, and C, with the sides AB and AC being the same length. Now note that there are two diretions which you might travel around the vertices: BAC and CAB. These give you a way to treat one triangle as two. Now note that the side BA of the first triangle euqals the side CA of the second triangle, and side AC of the fist triangle equals side AB of the second triangle. And of course the containing angle BAC is equal to the containing angle CAB, as they are really the same angle viewed in two different ways. Thus the corresponding parts are equal, and in particular the angle ABC in the first triangle equals the angle ACB in the second, which is the proposition to be proved.

If that makes your head hurt, you may wish to refer back to your copy of Euclid for his somewhat longer-winded but easier to follow proof.

Nowadays the phrase has entered something that might be called academic colloquial use, and is sometimes used to refer to any difficulty, so, for example, one reads of the pons asinorum of relativistic physics being an understanding of the constancy of the speed of light.