A term used by bakers to define the inside of a bread or pastry - its internal cell structure. Many factors can influence the cell structure of the crumb. For example, a high hydration dough that has not been over kneaded, has been properly shaped, and has sufficient yeast available at the time of baking to produce a good, final oven spring will generally result in a crumb that is open and airy with a lot of large, irregularly-sized and shaped holes (called alveoles).
Conversely, a dough with low hydration, rolled rather than hand shaped, and with low yeast levels at the time of baking (resulting in a poorer oven spring) will generally result in a crumb that is more bread like, with small, even-sized and tightly formed cells.
Added by: shipton-mill