An epistemic action is an action the purpose of which is to gain information. Low level examples include turning one's head to look at something you glimpsed from the corner of your eye, or opening a box to see what's inside. At a higher level you might look something up on the web, or go to a museum. This is important in computer vision as many ambiguous images resolve quickly if the veiwpoint is changed even slightly by movng camera position, a process known as active vision.
Used in Chap. 15: page 232