In the 60's, Japanese walking clubs defined "10-thousand steps per day" to be the benchmark for being an "Active" adult. At the time, Japanese research estimated the average person walked only 3,500 - 5,000 steps each day.
The distinction originated in 1965 when the watch company, Yamasa Tokei, introduced the first pedometer (counts steps) with the slogan, "Let's all walk 10,000 steps a day!" From there, 10-thousand steps became synonymous with the minimum steps it takes to be reasonably healthy.
All these years later, following the wake of the popularity of personal monitors from FitBit, VivoFit, Jawbone and others, 10-thousand steps is still a standard threshold to be Active.