What is another word for liquid substance?

There isn't one perfect synonym for "liquid substance," as it's already quite specific. However, depending on the context, you could use:

More formal:

* Fluid

* Liquor

* Solution

* Solvent (if it dissolves something)

More informal:

* Drink (if it's something you can drink)

* Beverage (if it's a drink)

* Juice (if it's a fruit-based drink)

* Goo (for a thick, sticky liquid)

More specific:

* Water

* Oil

* Blood

* Ink

Ultimately, the best word to use depends on what you're trying to communicate.