ordinarily; usually; in normal times
Adverbial noun meaning 'in ordinary days' or 'normally'; often used with の to modify nouns or as a standalone adverbial phrase. Somewhat formal or written style.
I usually get up early, but I overslept today.
He is usually calm, but unusually, he was angry.
Compound of 平 (へい, 'ordinary') and 生 (ぜい, 'life'), with the topic particle は fossilized into the word. The reading ぜい for 生 is a kan'on reading.