Making a stipulative definition

Maybe:

Let us stipulate that X means ...

or, if you are elsewhere using the first person singular,

I stipulate that X means ...

Note: I suppose a "stipulative definition" is concept known in philosophy? And you have to include the word "stipulate"? In mathematics, I would just say things like

Let X mean...
Let X be...
Let X = ...

and, more flexibly,

... is called X if ...
... is said to be X if ...

What about this:

... X, stipulatively defined as Y, ...

or

... X, stipulated to mean Y, ...

or

... X, stipulated as Y, ...

Stipulative definitions are a type of definition but are not always identified by using the word stipulate. There are a whole host of signals commonly used.

All of these are examples of stipulative definitions:

• Suppose we say that to love someone is to be willing to die for that person.
• Take "human" to mean any member of the species Homo sapiens.
• For the purposes of argument, we will define a "student" to be "a person under 18 enrolled in a local school".
• Let X = any integer

Indeed the examples in OP ― which aren't wrong ― probably sound awkward only because you're used to reading so many alternative ways of calling out a stipulative definition.