Saltire – Wikipedia

The diagonal cross (decussate cross) or X mark is called “saltire” in heraldic and vexillological contexts. A black diagonal cross was used in an old European Union standard as the hazard symbol for irritants (Xi) or harmful chemicals (Xn). It indicated a hazard less severe than skull and crossbones, used for poisons, or the corrosive sign.

https://en.wikipedia.org/wiki/Saltire