A fold of skin of the upper eyelid
that partially covers the inner corner of the eye
. This attribute is what most often is used to distinguish Asians
Some anthropologists suggest that since east Asians evolved in harsh winter climates about 40,000 years ago, the epicanthic fold is an adaptation against the glare of the sun on ice.
tongpoo says epicanthic fold is prevalent in Mongolian descendants rather than Asians in general. That's why not all Asians have it (especially in the south).