40 CFR 144.3
Fluid means any material or substance which flows or moves whether in a semisolid, liquid, sludge, gas, or any other form or state.
Injection well means a "well" into which "fluids" are being injected.
Well means: A bored, drilled, or driven shaft whose depth is greater than the largest surface dimension; or, a dug hole whose depth is greater than the largest surface dimension; or, an improved sinkhole; or, a subsurface fluid distribution system.
Well injection means the subsurface emplacement of fluids through a well.
N.H. Admin. Code Env, Env-Wq, ch. Env-Wq 400, pt. Env-Wq 404, app D