The short answer
A mitzvah is a commandment in Judaism. The word can also mean a good deed in everyday speech, but the classical meaning is stronger: an act Jews are obligated to do, or avoid doing, within the framework of Torah, rabbinic interpretation, covenant, and Jewish law.
People say, "Do a mitzvah," and often mean "do something nice."
That is not exactly wrong, but it is not the core meaning. In Judaism, a mitzvah is first of all a commandment. The Hebrew word refers to an obligation rooted in Torah and later Jewish legal tradition. That is why a mitzvah is more than a kind gesture someone happens to admire. It belongs to a framework of duty, law, covenant, and divine expectation.
The softer modern meaning came later.
In classical Judaism, a mitzvah is a commandment
Britannica gives the clearest short definition: a mitzvah is a commandment, ordinance, law, or statute contained in the Torah and therefore binding on practicing Jews. My Jewish Learning says the same thing directly in reader-friendly terms. A mitzvah does not originally mean "good deed" in the vague ethical sense. It means commandment.
This matters because it changes the emotional force of the word.
If mitzvah simply meant "kind act," Judaism would be recommending virtue. When mitzvah means commandment, Judaism is doing something stronger. It is saying that certain actions are required, beyond admiration or personal approval.
That is why the word can feel warm in casual speech and demanding in religious context. A mitzvah may be joyful, but its meaning does not depend on the mood around it. The act belongs to a commanded way of life.
The tradition famously speaks of 613 mitzvot
My Jewish Learning notes that Jewish tradition commonly counts 613 mitzvot derived from the Hebrew Bible. Britannica also points to this classical framework and explains the standard division between positive commandments, things one must do, and negative commandments, things one must not do.
That count has become one of the best-known features of Jewish legal thought, even though ordinary Jews do not spend most of their lives ticking through a numbered list.
The point of the number reaches beyond arithmetic. It expresses the idea that covenant extends deeply into life. Mitzvot shape action, speech, ritual, time, food, money, sexuality, care for others, and the relation between private conduct and communal obligation.
That breadth is easier to see once readers connect the word to pages such as What Is Torah?, where commandment begins in revelation, and to practice-heavy topics such as What Is Kosher?, where obligation shows up in ordinary eating.
It also helps to place mitzvah beside tzedakah, because that comparison keeps the article from treating commandment as only ritual; Jewish obligation also organizes money, justice, and care for other people.
Positive and negative mitzvot work differently
The traditional split between positive and negative commandments matters because it keeps the word from becoming vague.
Some mitzvot require action: give charity, honor parents, observe sacred times, or perform ritual acts. Other mitzvot prohibit action: do not steal, do not bear false witness, do not use dishonest weights, do not violate particular boundaries of Jewish law. Both kinds are commandments, but they shape life in different ways.
That distinction helps explain why mitzvah is more than a synonym for kindness. Some mitzvot feel obviously ethical to a modern reader. Others are ritual, legal, dietary, or time-bound. The word covers a wider religious field than the English phrase "good deed" can hold.
Why people say mitzvah to mean a good deed
The popular English use did not come from nowhere.
Britannica notes that in broader usage many Jews treat good deeds as mitzvot because such acts are understood as expressions of divine will. That is why someone might praise a charitable act by saying, "It's a mitzvah." My Jewish Learning makes the same point while warning that this looser use can obscure the term's legal meaning.
So the colloquial use is not nonsense. It is just broader and softer than the formal one.
It reflects the fact that Jewish life does not separate morality from commandment as neatly as modern secular language often does. Acts of generosity, charity, and care can be discussed as mitzvot because they are kind and also religiously meaningful obligations.
Not all mitzvot are equal in the same way
Another misconception is that if there are 613 mitzvot, they all function identically.
Britannica points out that Jewish tradition recognizes differences in weight and scope. Some commandments are direct biblical requirements. Some later practices, while authoritative in Jewish life, derive from rabbinic enactment or custom rather than from the Torah in the narrowest sense. Some mitzvot apply only in particular times, places, or social conditions.
That is why learning what a mitzvah is quickly turns into a broader question: which kind of mitzvah, according to which source, for whom, and under what circumstances?
This is one reason mitzvah cannot be reduced to a sentimental slogan. The term belongs inside a serious legal and interpretive tradition.
Bar mitzvah and bat mitzvah preserve the older meaning
The phrases bar mitzvah and bat mitzvah also help clarify the word.
In common English, people often use those phrases for the ceremony or party. In Jewish meaning, they refer to becoming obligated in the commandments. A Jewish child who reaches the relevant age is not becoming a "good deed." The child is becoming a person counted as responsible for mitzvot in a new way.
That is why the phrase keeps the commandment meaning close to the surface. The celebration may be social, emotional, and communal, but the concept underneath it is obligation.
The word still matters because it ties ethics to obligation
Modern people are used to thinking in terms of preference, values, and identity.
Mitzvah introduces a different grammar. It asks what feels meaningful or admirable, and what is required. That can apply to prayer, Sabbath observance, charity, honesty in business, care for parents, treatment of strangers, and countless other parts of life.
This is one reason the word has lasted even outside strictly observant circles. It carries the sense that good action is more than elective preference. It places moral and ritual life under the sign of responsibility.
That responsibility is what the casual English use can hide. Calling something "a mitzvah" may sound warm, but the older word asks a harder question: is the act only admirable, or is it part of what Jewish life requires from a person?
The shortest accurate answer
If someone asks what a mitzvah is, the shortest accurate answer is this:
A mitzvah is a commandment in Judaism, though the word is also used more loosely for a good deed because acts that do good are often understood as fulfilling Jewish obligation and divine will.
That answer keeps both meanings in view without confusing which one came first.