I know it's been a while since I've posted one of these (sorry about that!).

This is classically defined as a fall in systolic blood pressure >10mm Hg on inspiration, which is an exaggeration of the normal response. Classically seen in pericarditis.

Here's a related mnemonic from First Aid:
PERICarditis
Pulsus paradoxus
ECG changes (PR-segment depression in precordial leads, low voltage, diffuse ST segment elevation)
Rub (friction)
Increased JVP
Chest pain