A biohazard was something that was dangerous to organic matter.
The Enterprise NX-01 featured a bio-hazard locker for the storage of such matter. (ENT: "Impulse")
Plasma coolant was considered a biohazard because it would react violently with organic matter, liquefying it on contact. (Star Trek: First Contact)
In 2376, Harry Kim commented that a dirty plate left in the Delta Flyer's replicator was a biohazard. (VOY: "Memorial")