Basis
The gap between a futures price and the spot price of the same asset — the premium you collect in a spot-futures trade.
Basis is futures price minus spot price. A positive basis (contango) means the future is richer than spot, usually because leveraged demand is long; a negative basis (backwardation) means the opposite.
On a dated future, basis is a return with a deadline: the contract must equal spot at expiry, so buying spot and shorting the future locks the gap, annualised over the days remaining. On a perpetual there is no expiry, and funding plays the role of forced convergence instead.
Basis widens during strong moves and in the days after a listing, and it compresses when leverage unwinds. That is why a basis trade is entered on the extreme and closed on the mean, rather than held indefinitely.