pacer

Short definition from Wiktionary

Noun

1. 1
   One who paces.
2. 2
   In harness racing, a horse with a gait in which the front and back legs on one side take a step together alternating with the legs on the other side; as opposed to a trotter.
3. 3
   A pacemaker (one who sets the pace in a race).
4. 4
   A timer in an electric toothbrush that indicates how long the user should spend brushing in each part of the mouth.
5. 5
   A mechanical pencil.
6. 6
   A British lightweight railbus diesel multiple unit of class 140, 141, 142, 143 or 144.

Proper noun

1. 1
   Acronym of Public Access to Court Electronic Records, an electronic public access service for United States federal-court documents.