Illustrated Glossary of Organic Chemistry

Ipso hydrogen: In an electrophilic aromatic substitution reaction, the hydrogen bonded to the arenium ion carbon that is the site of attack by the incoming electrophile. In a traditional electrophilic aromatic substitution reaction, the ipso hydrogen is the hydrogen replaced by the new substituent (from the attacking electrophile) in the final product.

In this electrophilic aromatic bromination reaction, the hydrogen replaced by bromine in the final product was the ipso hydrogen of the arenium ion.