What is an Animal?

The general definition of an animal is given by Webster (http://www.merriam-webster.com/dictionary/animal) as the following:  

• a living thing that is not a human being or plant
• any living thing that is not a plant
• a person who behaves in a wild, aggressive, or unpleasant way

The full definition is given as follows:

Any of a kingdom (Animalia) of living things including many-celled organisms and often many of the single-celled ones (as protozoans) that typically differ from plants in having cells without cellulose walls, in lacking chlorophyll and the capacity for photosynthesis, in requiring more complex food materials (as proteins), in being organized to a greater degree of complexity, and in having the capacity for spontaneous movement and rapid motor responses to stimulation

The full definition takes more of a biological standpoint, whereas the general definitions agree that plants are not to be considered animals, but that humans may or may not be considered so. For my in class writing, the characteristics that came to my mind when trying to answer the question were the ability to move, to breathe, to reproduce, to seek out sustenance(not photosynthesize it). In some respects, my thoughts align with the full definition, but I did not consider single-celled organisms or insects to be animals; I believe the average person does not view the ants outside steadily at work in their anthill as animals. What is an animal? I suppose it depends on who one is speaking to and what kind of answer that person seeks. The simple one or the full and involved definition.