Noun - Stroke rate or SR is the number strokes/second while swimming freestyle or other swimming strokes.
Example from About.com - Swimming:
Swimmer completes the 100m freestyle in 1:00, using 54 stroke cycles.
DPS = distance/# cycles used for that distance = 100/54 = 1.85 meters/cycle
SR = # cycles/time = 54/1:00 = .9 cycles/second
Seconds/Stroke = time/# cycles = 1:00/54 = 1.11 seconds/cycle