a77f2dca___General_Information_and_FAQs__Definition
[Term] Letterboxing [Definition] 
Another game, similar to caching, but without the use of a GPS. Letterboxes are listed on Letterboxing.org, and work much the same way as geocaches. Follow hints and riddles to find the hiding place. Typically involves using a personal rubberstamp to stamp log-book. Letterboxes also contain a special stamp with which you stamp your own log.
Geocachers of the Bay Area - General Information and FAQs
Definition