Bacteriostatic water
Also: BAC water
In plain language
Sterile water containing a small amount of preservative, used to dissolve powdered peptides for injection.
Technical definition
Sterile water for injection containing 0.9% benzyl alcohol as a bacteriostatic preservative, allowing a multi-dose vial to be accessed repeatedly without microbial growth.
The benzyl alcohol in bacteriostatic water inhibits bacterial growth, which is why it is preferred over plain sterile water for vials that will be used across multiple doses over several weeks.