What do we call a part of the body with a special function?

Organs are specialised tissues that functions on a specific task to maintain and sustain the survival of an organism.

Cells when grouped form tissues, when group of tissues are formed together they make organs, some organisms skip from organs to organs systems to form an existing organism as a whole. Describing how is the cells group is the process of multiplication of cell or the so-called cell division, mitosis.This mechanism of each cell produces another cell that binds together and produces again, repeatedly, to be a larger system called now the tissue as explained earlier. How? Through cell division called, mitosis. Excerptmeiosis –cell division specifically in the sex cells. 

This is giving me a headache i have asked others and they don’t know it either