The purpose of this paper is to characterize some definitions of function in engineering and philosophy from an engineering point of view. To do this, this paper firstly discusses some fundamental kinds of function based on ontological distinctions. Next, based on them, we characterize our definition with justifications from the engineering view and present an ontological model of artifact function along the product life-cycle. Then, we try to characterize a philosophical definition of biological organs using the model. Next, we discuss function of a non-biological (non-organic) natural thing. Lastly, we characterize functions of an artifact, a biological organ and a non-biological natural thing based on another ontological model along the evolution of creatures.