What is another word for first law?

There isn't a single, perfect synonym for "first law" because its meaning depends on the context.

Here are some possibilities, depending on what you mean:

For laws in a legal system:

* Fundamental law

* Primary law

* Constitutional law (if it's a law of the highest authority)

* Basic law

* Cardinal law

For laws in science or other fields:

* Axiom

* Principle

* Postulate

* Fundamental law

* Foundational law

For specific "first laws" in a series:

* Initial law

* Starting law

* Precedent law

* Initial principle

To give you a more accurate alternative, please provide more context. For example, are you talking about the first law of thermodynamics? The First Amendment of the US Constitution? The "first law" of a fictional universe?