In botany and zootomy, an \organ is a group of tissues which perform some function. Common animal organs include the heart, lungs, brain, eye, stomach, spleen, pancreas, kidneys, liver, intestines, skin, uterus, bladder, etc. Common plant organs include the stalk, stem, leaf, roots, and stamen. A group of related organs is an organ system.
Organelles are analogous sub-cellular structures.