Torque is a moment: force times lever arm. Fastener specs, motor datasheets, and service manuals mix its units constantly — newton-meters in metric documentation, foot-pounds on US torque wrenches, inch-pounds in small-fastener and aviation specs, kgf·m in older metric and Japanese service literature — with the NIST SP 811 factors bridging them: 1 ft·lbf = 1.355818 N·m, 1 ft·lb = 12 in·lb exactly, and 1 kgf·m = 9.80665 N·m exactly (≈7.233 ft·lb). Unlike a flow or an energy, torque is honestly signed — tightening versus loosening, opposite moment directions in a free-body diagram — so a negative value converts normally rather than flagging an error.
On a rotating shaft, torque and speed multiply into power: P = T × ω, which in working units reads HP = T(ft·lb) × RPM ÷ 5252.113 (exactly 33,000/2π, since one horsepower is 33,000 ft·lb of work per minute by definition) and kW = T(N·m) × RPM ÷ 9549.297. The 5252 constant explains a piece of dyno-chart folklore — the torque and horsepower curves always cross at 5252 RPM — and at the common industrial shaft speed of 1,800 RPM the shortcut is HP ≈ torque ÷ 2.92.
At the fastener, torque is a means rather than the end: the wrench setting exists to develop clamping tension, connected through the empirical relation T = K·F·d in which the nut factor K encodes how much of the applied twist is eaten by thread and under-head friction before it becomes bolt tension. The same 218 N·m means very different preloads on dry versus lubricated threads — which is why torque specs always name their friction condition.