Just to add to what others have said, the last time I rooted an Android phone (Pixel 6), I had to run:

    fastboot flashing unlock_critical

So it seems that at least in some contexts, "unlock" can refer to the bootloader (sense 2 above).

Also I wasn't as clear as I intended to be with sense 3 above - it might also mean temporarily screen-unlocked, as in "can you please unlock your phone so I can enter my number?".