Challah, the plaited white loaf that we eat at Shabbat meals, is among the best known Jewish religious objects. Last time I was in New York, I passed a café on Broadway advertising bacon and eggs with challah rolls. As usual the US is far ahead of us in catapulting cultural Judaism, shorn of its religious roots, into the mainstream.
In the Bible, challah is the portion of bread that is set aside and given to the priests to eat (Numbers 15:19-20). The mitzvah of separating challah applies to the five grains, wheat, barley, spelt, oats and rye. The rabbis calculate that more than 1.75 kg of dough baked at one time must have challah taken from it.
According to the Torah, the mitzvah of challah is relevant only in the Land of Israel. However, the rabbis instituted it outside Israel as well, so that the mitzvah should not be forgotten. Nowadays, the custom when baking a sufficiently large amount of bread is to break off a small piece of dough the size of an olive and burn it.
According to the Encyclopaedia Judaica, one derivation is from the Hebrew root challal, meaning "hollow," suggesting a rounded loaf; another possibility is that it comes from the Akkadian ellu, meaning "pure," referring to the bread's sacred use.
The medieval compendium, Sefer Hamitzvot, says that the reason for challah, aside from the obvious one of supporting the priests, is that since our lives depend on bread, God gave us a mitzvah to fulfil whenever we bake, so that our bread may be blessed continuously.