A Technical Standard Order (TSO) is a minimum performance standard issued by the United States Federal Aviation Administration for specified materials, parts, processes, and appliances used on civil aircraft. Articles with TSO design approval are eligible for use on the United States type certificated products. The TSO authorization or a letter of TSO Design Approval does not necessarily convey approval for installation. See CFR Part 21 Subpart O.
see excellent Honeywell overview at: http://www.honeywell.com/sites/aero/Aircraft_Certification3_C2287AD59-3F9D-6C2E-C5E2-86855F160770_H6B56709F-3C71-3B17-8FE6-B372A64AA28C.htm