Screw... This is a high quality wood screw, nickel-plated high tensile strength steel, at about 20x. These are what Nest supplies with its “Protect” fire/smoke/carbon monoxide alarms. Note the asymmetrical raised threads: one of them is higher and sharper than the other. Those were new to me, but I discovered they're called “high-low” threads, and provide several desirable characteristics for this sort of use. In particular, they're easier to screw in (I can attest to that) and harder to pull out – nice combination!