cracker a name first used in N. America, from the mid-18th century onwards, for a plain, unsweetened, dry, hard, bread product; thus corresponding to part of the domain covered by the wider English term biscuit (of which another major part belongs to the American term cookie). When crackers are broken into pieces they make a cracking noise, which accounts for the name.
Crackers may be leavened or unleavened. Those of the former sort were formerly baked by a particular method which called for a dough leavened with bicarbonate of soda (hence the term ‘soda cracker’) and left to stand until pockets of carbon dioxide formed in the mixture. When biscuits of this dough were placed in a very hot oven they rose quickly, giving the characteristic texture.