Base Peak
The most intense peak in a mass spectrum, assigned an arbitrary relative abundance of 100%.
Unit
Dimensionless
In depth
The base peak corresponds to the most stable or most readily formed fragment ion, so its identity is itself a structural clue. It is not necessarily the molecular ion, and in heavily fragmenting molecules it is usually a small, highly stabilised fragment.
Examples in the real world
Ethanol's base peak at m/z 31 from the resonance-stabilised CH₂=OH⁺ ion.