s24o is an abbreviation for...

s24o is an abbreviation for sub-twenty-four-overnight, a style of bicycle camping promoted by Grant Petersen of Rivendell Bicycle Works as an easy way to spend more time outside on a bike.Unlike conventional touring, the s24o encourages going on short (sub twenty-four hour) camping trips.

—http://en.wikipedia.org/wiki/Rivendell Bicycle Works#S24o