Bacteriostatic water is sterile water containing roughly 0.9% benzyl alcohol as a preservative. The alcohol is bacteriostatic rather than bactericidal: it suppresses bacterial multiplication rather than sterilising the contents, which is what makes repeated entry into the same vial practical.
It differs from sterile water for injection, which contains no preservative and is intended for a single entry, and from water for injection (WFI), a bulk pharmaceutical-grade water produced to a defined endotoxin specification. The three are not interchangeable, and the difference is the preservative rather than the purity of the water itself.
Because the preservative is a solvent in its own right, it is worth noting where a protocol is sensitive to it. Diluent choice is part of the experimental record, not an afterthought — the reconstitution step should be logged with the volume and the diluent used.