A Single Tear is a cliché that generally occurs when a Mary Sue angsts. It is meant to be a more dignified way of showing sadness than full-on sobbing, which tends to leave characters with unattractive red and puffy eyes. Often, it is described with very urple prose, something along the lines of "a pearl-like drop trickled down her silky cheek."
When a male character does it, it may be a Manly Tear, because man's man manly men are obviously too full of testosterone and y-chromosomes to outright cry.
However, the Single Tear is also used in many canons, such as Torchwood, Supernatural (especially Dean) and the movieverse of The Lord of the Rings. The Inheritance Cycle is especially guilty of it. In these continua, a more appropriate charge might be that of ripping off canon characters.