In Jewish usage, the Written Law usually refers to the Torah in the narrow sense: the Five Books of Moses. It is called written not because it is the only sacred teaching, but because it is distinguished from the Oral Law, the interpretive tradition that explains how the text is to be understood and applied.
That distinction is central to rabbinic Judaism.
The Written Law usually means the Torah
Jewish Virtual Library states the classical definition plainly: the Written Law is the Torah, the five books of the Hebrew Bible associated with Moses. Britannica's broader Torah article supports the same point while noting that Torah can also be used in wider senses.
That narrower meaning matters here.
When Jews speak of the Written Law in contrast with the Oral Law, they usually mean Genesis, Exodus, Leviticus, Numbers, and Deuteronomy as the foundational written revelation.
It is called written because Judaism also speaks of an Oral Law
The phrase only makes full sense alongside its counterpart.
Britannica explains that for many Jews the traditions and interpretations transmitted orally alongside the written text came to be understood as part of divine revelation as well. Jewish Virtual Library makes the same distinction in more traditional terms, presenting the Written Law and Oral Law as inseparable parts of Torah in the broad sense.
This is why the Written Law is not usually treated as self-sufficient.
The text is foundational, but the living practice of Judaism depends on interpretation, precedent, and application. The Written Law provides the words. The Oral Law provides much of the method.
The Written Law is textual core, not the whole of Jewish law
This is the main modern misunderstanding to avoid.
If the Written Law were the whole of Jewish law, many commandments would remain too terse or ambiguous to govern lived practice. That is why the Oral Law, and later halakhic tradition, became so important. The Written Law anchors Jewish covenantal life, but it does not remove the need for interpretation.
That is also why the Written Law still carries singular prestige. It is the textual core around which the rest of the tradition gathers.