A number has consecutive digits when every neighbouring pair of digits differs by exactly 1 — each step moves up or down by a single place. This includes the obvious runs like 1234 and the reverse 4321, but also zig-zag walks that change direction, such as 12321, 234321, or 12323456545.
Think of it as a walk along the number line of digits 0–9 that takes a single step at every move. Pure ascending or descending runs (1234, 9876) are the special case where the walk never turns around.
It's a close cousin of the [[stepped-digits]] family, which uses a constant step of 2 or more instead of 1.