Illustrated Glossary of Organic Chemistry

Relative abundance (relative intensity): The y-axis of a mass spectrum. Quantifies the amount of an ion produced in relation to the amount of the most abundant ion (the base peak).

In this simulated mass spectrum, the ion of m/z = 30 has relative abundance of 100%, so it is the base peak. The ion of m/z = 50 has a relative abundance of 30%, meaning the mass spectrum contains 30% as many ions of m/z =50 as m/z = 30 (the base peak). Note that this does not mean 100% of the ions produced have m/z = 30 and another 30% have m/z = 50. This is not possible because 100% + 30% = 130%, and it's not possible to produce more than 100%.