Saltire v. : A St. Andrew's cross, or cross in the form of an X, -- one of the honorable ordinaries.

Example:
Why are some Scottish flags (Saltire) light blue and others dark blue. What is the difference?

Taxonomy
saltire, flags, Cross, arms, St Andrew's Cross, Roes, The Flag, free encyclopedia, Christian symbol, Flag of Scotland, coins, the cross, royal arms of Scotland, crux decussata, Heraldry, couped, coat of arms of Nova Scotia, Nova Scotia.

Defination from the web:
saltire, Saint Andrew's Cross, or crux decussata (though it is never called the last in heraldry), is a heraldic symbol in the form of a ... The saltire appears on vexilla that are represented consistently on coinage of Christian emperors of Rome, from the fourth century. ...