Definition
A continuous helical (spiral) ridge cut or formed onto the surface of a cylindrical part, allowing two parts to be joined by turning one against the other. Threads are defined by their pitch (distance between ridges), diameter, and form (the shape of the ridge), and may be either external (on a bolt or stud) or internal (inside a nut or tapped hole).
Plain English
The spiral ridges on a screw, bolt, or inside a nut that let two parts twist together and hold tightly.
Context Anchor
Seen during aircraft maintenance when inspecting bolts, nuts, screws, spark plugs, fittings, and other parts that fasten or connect by turning.
Derivation
From Old English thrǣd, meaning a fine cord or strand. The mechanical sense came from the visual resemblance of the spiral ridge wrapping around a bolt to a thread wound around a spindle.
Why Pilots Care
Proper threads keep critical components from loosening under vibration and flight loads; damaged threads can cause structural or engine failure.
Analogy
A bottle cap and bottle neck have matching threads. When the spirals line up, the cap turns on smoothly and tightens; when they do not, it binds or goes on crooked.
Intuition Check
Do not read thread here as sewing thread or an online discussion. In aircraft maintenance, a thread is the spiral shape on a part that lets it screw together with another part.
Example Sentence 1
The mechanic rejected the bolt because the threads were damaged and would not hold torque properly.
Example Sentence 2
A stripped thread in the propeller hub fitting required replacement of the entire part.