is any anatomical structure that is composed of at least two different tissue types has recognizable structural boundaries, and has a discrete function diffeent from the structures around it

An Organ

Explanation:

An organ is composed of a group of different tissues that carry out one or more discrete function. In general the body organization includes the cells coming together to form tissues, which also forms the organs and ultimately forms the functioning body system. Example of an organ includes the liver, and the brain