Kashrus is the body of Jewish law governing what may be eaten and how it has to be produced. It is wider than most summaries of it, and almost none of it is visible in a finished product — which is the whole reason kosher certification exists.
Kashrus, kashrut and kashruth are the same word. So are kosher and kasher. They are כשרות and כשר written out according to Ashkenazi or Sephardi and Israeli pronunciation, and nothing halachic turns on which you use. This site writes kashrus throughout; read it as kashrut if that is the spelling you know.
The word means fitness. Something kosher is fit for its purpose, which is why English borrowed it for anything above board. The slang sense is a straight translation of the Hebrew rather than a corruption of it.
It is not a list of forbidden foods
No pork, no shellfish, no cheeseburgers — the popular summary names three outcomes and misses the shape. Kashrus is a sequence of independent questions, and a food has to pass every one of them. A permitted species prepared wrongly is not kosher. A flawless ingredient list cooked by the wrong person is not kosher. Each question below is answered separately, and failing any one is enough.
The questions, in the order they apply
1. Is the species permitted?
Mammals are decided by two physical signs — split hooves and chewing the cud — and both can be checked directly, so no tradition is needed to establish a mammal. Birds are the opposite case: the Torah gives no signs, names twenty-four species as forbidden, and the identification of those names has been lost, so a bird is eaten only where there is a mesorah. Fish are decided by fins and removable scales.
Insects are forbidden too, which in practice turns out to be a produce question rather than a species one — nobody is choosing to eat them.
Do animals need a mesorah? · Which birds are kosher · Which fish are kosher · What mesorah is
2. Was the animal slaughtered and prepared correctly?
A permitted species is not yet permitted meat, and this is where most of the work is. The animal must be slaughtered by shechita, then inspected for the defects that render it treif — not every animal of a kosher species passes. Then comes nikkur, the removal of the forbidden fats, the blood vessels and the sciatic nerve, and melicha, salting to draw out the blood.
Nothing in this stage is optional and none of it is visible afterwards. It is why “the animal is kosher” settles so much less than people expect.
What shechita involves · What invalidates shechita · What makes an animal treif · Nikkur and the forbidden fats · Melichah
3. Has meat met milk?
Meat and milk may not be eaten together, cooked together, or benefited from together — three separate prohibitions, not one. In a kitchen that means separate vessels, separate handling, and a waiting period after meat. Hard cheese carries a rule of its own.
Waiting between meat and dairy · Meat and dairy at one table · Marking vessels · After hard cheese
4. Who made it?
A set of rabbinic enactments turn on who did the work rather than what went in: bishul akum for food cooked by a non-Jew, pas for bread, gevinas yisroel for cheese, and the rules on wine and on milk. These are the categories that most often decide that a product with an entirely kosher ingredient list is still not kosher — and they are invisible on a label by definition, because they are not about ingredients at all.
Bishul akum · Pas yisroel and pas palter · Cheese and gevinas yisroel · Cholov yisroel and cholov stam · Why wine is different
5. What grew it?
Produce carries its own set. Insects are the practically demanding one — a vegetable is a kosher food that can be made non-kosher by what is living on it. Orlah forbids the fruit of a tree's first three years, and chodosh restricts new grain. In the Land of Israel the agricultural mitzvos — terumos and maasros, shemittah — apply on top.
Checking vegetables for insects · Orlah · Yoshon and chodosh
6. What was it cooked in?
A vessel absorbs the taste of what was cooked in it, and gives it back. That single principle generates the laws of kashering — hagalah, libun and irui, each matched to how the vessel absorbed — and it is why a kosher kitchen is a question about equipment as much as about food. Separately, vessels acquired from a non-Jew require tevilah before use.
Hagalah, libun and irui · When a vessel becomes forbidden · Tevilas keilim
7. And then Pesach
Pesach is less a seventh category than a stricter re-run of the others. Chametz is forbidden in any quantity, so the mixture rules that resolve most of the year do not help; the kitchen is kashered rather than merely cleaned; and ingredients that pass all year are re-examined.
Kosher for Pesach basics · Chametz mixtures · Kashering the kitchen for Pesach
Why this became a certification question
The problem is an old one, and openly acknowledged. The laws of food kashrus are many and intricate and demand real knowledge, every Jew is obliged to know them well enough not to stumble, and the pressures of ordinary life mean few people have the time to learn them in full detail. On top of that, the field does not sit still: there is scarcely a day without some development in food production that needs a fresh halachic answer.
Industrial food made that gap structural. A finished product shows you none of the seven questions above. An ingredient panel names substances — not their sources, not who made them, not what the line ran before, not what the processing aids were. Reading a label carefully cannot answer a single one of the questions on this page.
That is the gap a hechsher closes, and it is old enough to be a chapter of the halacha in its own right: the rules on kosher markings for things that need certification sit alongside the rest. A symbol is not a marketing claim about an ingredient list. It is a statement that somebody with the training checked the things you cannot see.
Why a kosher species may not be certified · How certification works
Where to go from here
This page is a map, not an answer to any particular question. Every heading above has pages underneath it that go properly into the detail, and the halacha index indexes all of them. For a specific product rather than a principle, the is-it-kosher answers answers the questions people actually ask.