It will likely be Tick.
Like NL, I use the name Tick internally for the unit of a timer. Mine runs at 1/60 seconds. It's Allegro 5 functionality, and the A5 documentation occasionally calls the units Ticks, but the API consistently calls the units "timer count". Thus, in the long term, I can refactor away from my usage of Tick (for the A5 timer unit) to Tick for number of physics updates.
The word Tick is nowhere exposed in the UI, and thus would be fresh, no popular prior meanings. And Tick is certainly not the rendered graphics frame.
Frame: Has the benefit of existing other words, e.g., framestepping. Tickstepping is made-up. I like to say "rewind" for framestepping backwards, nice and short, but many people say framestepping, too.
Step: Possible, but dangerously ambiguous because walking lix also step forward. "Assign basher after the next step" is ambiguous.
Count: Alone, it's too nondescriptive. Count of what? Physics count seems longer than necessary, Phycount has the same problem as Phyu.
Move: I don't like it much. There is no user input in most physics updates. Move would rather be what players do, and I use Ply internally for a player action, which is either assignment or nuke. The replay tweaker is a ... replier. >_>;;
Generation: It's the common word in cellular automata for the result after a physics update, but that's because cells appear and die, whereas lix would exist across thousands of generations.
-- Simon