Question: "what does face detection mean? i mean, i know it's to find a face in an image, but what does it mean 'by definition'?"
Answer: Right! first things first! Face detection is a computer vision technology that determines the locations and sizes of human faces in arbitrary (digital) images. It detects facial features and ignores anything else, such as buildings, trees and bodies. Face detection can be regarded as a specific case of object-class detection. In object-class detection, the task is to find the locations and sizes of all objects in a digital image that belong to a given class. Examples include upper torsos, pedestrians, and cars.