Spread
The gap between the highest price anyone is currently willing to pay and the lowest price anyone is currently willing to accept.
The spread is the immediate cost of a round trip: entering and exiting at once loses roughly the spread, before fees. Market makers earn it as compensation for holding inventory and for the risk of trading against better-informed participants, so it widens when volatility rises or when few participants are quoting. Because digital assets trade continuously across many unconnected venues, spreads differ from venue to venue at the same instant and typically widen during overnight and weekend hours when professional firms reduce activity, a pattern with no direct equivalent in an equity market that closes. Quoted spread is what is displayed; effective spread is what a trade actually pays after crossing.
In practice
A widely traded pair on a large venue commonly shows a spread of a fraction of a percent, while a newly listed token on the same venue can show a far wider one.
The common misunderstanding
A tight spread does not by itself mean a market is deep; only a small quantity may be available at the best quote.