Bacteriostatic water
In short Water containing 0.9% benzyl alcohol, which limits bacterial growth in a vial opened several times.
Bacteriostatic water is sterile water with a small amount of benzyl alcohol added as a preservative. The alcohol does not kill bacteria outright; it stops them multiplying, which is what "bacteriostatic" means. That makes it suitable for multi-use vials that are punctured more than once in the laboratory.
It is the most common diluent for reconstituting lyophilised research peptides. It differs from sterile water, which has no preservative and is meant for single use, and from sodium chloride 0.9% (saline), which adds salt.
Benzyl alcohol is not inert in every experimental system. Some cell-based assays are sensitive to it, and in those cases a preservative-free diluent may be the better choice. The decision belongs to the protocol.
UKPeps bacteriostatic water comes in 10 ml vials and is a laboratory reagent only. It is not suitable for injection or for any human or veterinary use. Its certificates are in the library under "Bacteriostatic water".