Tarp: A large sheet of waterproof or water-repellant canvas, polyethelene, plastic (including PVC), and even cloth, usually with grommets at each corner and along each outer edge.
TFF (Time To First Fix): This is the amount of time it will take your GPS receiver to make its first position fix after it has been in the off position for over a month, lost its memory, or been moved for over 300 miles without an interim fix. Typically, this won't amount to more than 15 minutes.
TOG: The unit of thermal insulation of clothing. Any item of clothing or bedding (including sleeping bags) can be given a Tog rating. The Tog rating can be directly translated into a comfort temperature range. If you sleep warm or cold you may want to consider a slightly different rating. Tog values will seem low on sleeping bags compared to your duvet at home as sleeping bags are far more efficient than duvets.
Tongues: Not only provide padding for the top of the foot, but gussetted tongues keep out grit and debris between the sock and liner. Asymmetric tongues add support and comfort.
TPU: Thermoplastic polyurethane (TPU) is a versatile elastomer similar in property to both rubber and plastics. It is elastic, can be processed by heat and is extremely durable. Since no solvents are needed in its production TPU is an excellent alternative to PVC with a reduced impact on the environment.
Twistlock: A locking karabiner where the gate is locked with a spring-loaded clip.