What is basis in crypto
The perpetual price minus the spot price, as a percentage of spot. The basis is positive when the perpetual trades above spot.
In the product
On the Funding rate page in Z Market Lab, the Spot-perp arbitrage tab has a Basis column: the spread between buying spot at the best ask and selling the perpetual at the best bid, as a percentage of the spot price. Rate overview also shows the Long spot / Short perpetual basis.
How to read it
With a positive basis, buying spot and shorting the perpetual captures that spread at entry. The basis can then narrow or widen further. See What are basis and spread convergence.