1. (in surgery) An individual whose tissues or organs are transferred to another (the recipient). Donors may provide blood for transfusion or a kidney or heart for transplantation.

2. (in genetics) A cell that contributes genetic material for insertion into another cell, for example to produce a transgenic cell by genetic engineering.

3. (in chemistry) A chemical species (e.g. a molecule, group, or atom) that donates electrons, atoms, or groups to another chemical species.