donor atom
A ligand may be a whole molecule, but only certain atoms in it actually touch the metal. Picture someone holding a rope: the rope is the ligand, but the hand gripping the metal is the donor atom. It is the specific atom through which the lone pair is handed over.
Precisely, the donor atom is the atom of a ligand that carries the lone pair and forms the coordinate bond to the metal. In water it is oxygen; in ammonia it is nitrogen; in chloride it is the chlorine; in carbon monoxide, perhaps surprisingly, it is the carbon, not the oxygen. A ligand with several donor atoms can bind through more than one of them at once: ethylenediamine donates through both of its nitrogens, and EDTA through two nitrogens and four oxygens. The donor atom's identity matters enormously — nitrogen, oxygen, sulfur, phosphorus, and carbon donors behave very differently, and the hard-soft acid-base idea explains which donors prefer which metals (hard oxygen and nitrogen donors favor hard small metal ions; soft sulfur and phosphorus donors favor soft heavy ones).
Naming donor atoms is also how chemists describe linkage and ambidentate behavior: the same ligand can sometimes bind through different donor atoms. The thiocyanate ion can attach through its nitrogen (named isothiocyanato) or its sulfur (thiocyanato), giving genuinely different compounds. Identifying which atom is the donor — and how many there are — is the practical key to reading a complex's structure, its denticity, and its isomerism.
Carbon monoxide binds metals through its carbon, not its oxygen — written M-C-O. This is why metal carbonyls are M-CO, and it makes chemical sense: the carbon end carries the more available lone pair and can also accept electron density back from the metal.
Carbon monoxide coordinates through its carbon atom — the donor atom is not always the most electronegative one.
Do not assume the donor atom is the most electronegative atom in the ligand. In CO it is carbon, not oxygen; the binding atom is the one offering the most accessible lone pair and the best orbital match, not simply the one pulling electrons hardest.