Running cadence is the number of foot strikes per minute. Some definitions use steps per minute (total foot strikes, both feet), others use strides per minute (complete gait cycles, where each stride is two steps). This site uses steps per minute — the more common convention in running apps and watches.
The "180 steps per minute" rule of thumb originates from Jack Daniels' observation that elite runners at the 1984 Los Angeles Olympics averaged close to 180 spm. The observation has been widely repeated and simplified into a prescriptive rule, but Daniels himself noted this was an average of competitive runners at race pace, not a universal target.
The relationship between cadence and running economy is real but individual. Higher cadence at the same pace generally reduces vertical oscillation, reduces braking force, and shortens ground contact time — all of which can improve economy. But optimal cadence varies by runner height, leg length, running speed, and biomechanics.
For most recreational runners, increasing cadence by 5–10% from their natural rate has been shown to reduce impact forces and may reduce injury risk. Counting your cadence or using a metronome app for cadence training can be useful. But optimising cadence requires considering your specific biomechanics, not just matching a population average.