In biology, an organ is a group of tissues that work together in an organism. They specialise in particular vital functions. They create an organ system which, as a whole, is an organism.
These are examples of organs:
Organelles > Cells > Tissues > Organs > Organ systems > Organism
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Relationship of major animal lineages with indication of how long ago these animals shared a common ancestor. On the left, important organs are shown, which allows us to determine how long ago these may have evolved.