What is the difference between ETF volume and liquidity?

ETF trading volume is simply how many shares change hands in a period. Liquidity is broader: it's about how easily you can trade without moving the price, and what it costs to trade (mainly the bid-ask spread).

An ETF can have modest volume but still be liquid if the underlying holdings are liquid and market makers can efficiently hedge and create/redeem shares. Conversely, an ETF can have volume but still have wider spreads if the underlying market is stressed.

For practical trading, the spread is often the best quick check. If the spread is tight, liquidity is usually good. If it's wide, consider limit orders and trading during peak hours.