multiplying by one; original amount
Mathematical sense: multiplying by one, i.e., the same amount. Often used in contexts like 元の一倍 (the original amount).
This price is the same as the original.
This year's production was exactly the same as last year's.
Used adverbially in the fixed expression 人一倍 (more than others, exceptionally). Not used as a standalone adverb.
See also: 人一倍
He made an extra effort, more than anyone else.
It's said that in the old days, they used to double the price of this cloth to sell it.
一倍 now usually means the same amount (×1), while 二倍 means double (×2), even though 一倍 historically also meant 'double', causing confusion.
一倍 literally means 'one time as much' (i.e. the same amount), while 人一倍 is an idiom meaning 'more than others'.
Compound of 一 (one) and 倍 (times, -fold). Historically, 一倍 could mean 'double' (one time more), but in modern usage it usually means 'the same amount' (multiplying by one).