Cross margin is covered in full in the entry Cross-Margining, which explains how margin is shared across positions in clearing houses and on trading platforms. This page gives a short definition and explains how the term is used on crypto exchanges.
In cross margin mode, the whole balance of a trading account serves as collateral for every open position at once. Gains on one position can support losses on another, so a trader is less likely to be closed out by a single losing trade. The downside is that a large enough loss can drain the entire account, because the exchange can use all available funds before it carries out a forced liquidation. The alternative, isolated margin, assigns a fixed amount of margin to each position, capping the possible loss on that trade but making it easier to liquidate. Crypto derivatives exchanges usually let users choose between the two for each position in perpetual futures and other leveraged products.
"Cross margin" is the name used on most crypto platforms, while "cross-margining" is the broader term used in traditional clearing for offsetting risk across related positions or even across exchanges. Both describe sharing margin to reflect overall risk, a key topic in margin trading.