In multiple lead/start threads, each start is an individual thread- e.g., in a triple lead/3 start thread viewed from the side, the fourth thread crest is part of the same start as the first. Multiple start refers to the multiple thread starting points visible when looking at the thread edge-on, while multiple lead refers to the lead distance, equal to the thread pitch of a single lead screw multiplied by the number of individual starts. A triple-lead thread of 1mm pitch will advance 3mm per revolution. It will usually have three starts and three individual threads. That's why so much linear movement can be produced by focusing helicoids of fine pitch-they have lots of starts, therefore each thread has the lead of a much coarser single-lead thread.
It's possible for a multiple-lead thread to have fewer starts and individual threads than its lead; it will have blank area where the threads would normally be, will have less load-carrying ability, and will have to have clearance between the blank portions of mating parts. That makes possible a single thread which advances an uneven multiple; e.g., a 1.5X lead.
An interrupted thread is something else. It is a thread which has been removed around part of its circumference, along its axis. Its mating part usually has a complementary axial segment removed, allowing them to be mated linearly to nearly full depth, then turned a portion of a turn to reach full depth.
The late great Martin Forscher demonstrated that a screw thread camera and lens could be converted to a bayonet-action mount by interrupting the threads on a screw thread body and lens. The interrupted segments were lined up with the matching uninterrupted segments, the lens was then inserted as far as it would go, and a partial turn of the lens seated it fully. Both the body and lens had enough thread left to retain compatibility with unmodified lenses and bodies. It was particularly effective with screw thread cameras which had locks, like most Fujicas and later Mamiya/Sekors.