Not the same as volume. An instrument with high volume but a thin book can be illiquid in the way that matters. The practical measures are spread width, depth at the touch, and how quickly the book refills after a trade.
Liquidity is conditional. It is abundant when nobody needs it and vanishes when everyone does, which is precisely the failure mode behind forced-liquidation spirals.
Market makers are paid to supply immediacy, and the spread is that payment.