A quite prevalent synonym for sodium bicarbonate. This synonym is chiefly used in British English (as opposed to American English, where sodium bicarbonate prevails.)

IMHO, I think that sodium hydrogen carbonate makes more sense; after all, the formula is NaHCO₃, which is a sodium ion, a hydrogen ion, and a carbonate ion. "Bicarbonate" seems to suggest that there are two carbonate ions, which is not the case. However, the use of "bicarbonate" to describe a hydrogen and a carbonate together has become so ingrained that it is unlikely to go.