Bacteriostatic water
Bacteriostatic water is sterile water with a small amount of preservative added, which stops bacteria multiplying in it.
The preservative is usually benzyl alcohol at around 0.9 percent. Its purpose is to allow a container to be entered more than once without the contents becoming a growth medium.
That multi-entry property is the entire practical difference from plain sterile water, and it is why the two are not interchangeable for anything intended to be used more than once.
For example
Sold most commonly in 10 mL and 30 mL vials. A 30 mL vial contains 30 millilitres of liquid — a large number of small additions.
What it is not
Bacteriostatic water is not sterile water, and not distilled water. Sterile water for injection contains no preservative. It is also not a sterilising agent: it does not disinfect anything, and it cannot make a contaminated container safe.
Where this comes up: reconstitution