Donors are impurity atoms that have an extra electron in comparison to the substrate material. For silicon, which has four electrons in its outer orbital, any pentavalent impurity atom (such as phosphorous) is a donor as they have five electrons in their outer orbitals. These donor atoms are added to the substrate in one of many different ways and are typically at concentration levels of 10^12 - 10^18 atoms per cubic centimeter. Any higher concentration levels and you start to create an alloy versus a doped semiconductor.
Donors are pentavalent impurity atoms that give up an electron. They are added during the doping process to create a n-type semiconductor.
Electronic Devices : Conventional Current Version, 9th Edition by Thomas L. Floyd
Diffused impurities with five valence electrons are called donor atoms.
Electronic Devices and Circuit Theory, 11th Edition by Robert L. Boylestad & Louis Nashelsky