headway

Short definition from Wiktionary

Noun

1. 1
   Movement ahead or forward.
2. 2
   Forward motion, or its rate.
3. 3
   The interval of time or distance between the fronts of two vehicles (e.g. buses) moving in succession in the same direction, especially along the same pre-determined route.
4. 4
   Progress toward a goal.
5. 5
   The clearance beneath an object, such as an arch, ceiling or bridge; headroom.
6. 6
   A crossheading.