Therm 
Measurement unit of heat equal to 100,000 Btu. 

Thermal
Relating to heat. 

Thermal Coefficient of Expansion 
A factor specifying the expansion and contraction of a material as temperature rises and falls. 

Thermal Conductor 
A material that allows heat to flow through it 

Thermal Insulator 
A material that resists the passage of heat. 

Thermal Mass 
A substance of sufficient density to absorb heat, often used in passive solar heating systems. 

Thermal Spraying
An industrial process where metal is melted and sprayed onto a surface to form a coating. 

Thermocouple 
A thick copper wire with a sensor on the end, which cools and prevents the flow of gas to the burner if the pilot light goes out. 

Thermoelectricity 
Electricity produced by heating two different metals that are touching.

Thermometer 
A device for measuring temperature. 

Thermometer, Dry Bulb
Instrument which measure the amount of heat in the air by the expansion f a liquid in a graduated glass tube with a reservoir bulb at one end. Alternately, by use of a bimetallic strip which moves the indicator on a scale when heat causes the two metals to expand at different rates. Normally used for outdoor temperatures.

Thermoplastic 
A type of plastic, hard when cool, that is easily softened by applying heat. 

Thermoplastic-insulated Wire
Electrical wire with a thermoplastic covering. Designated as type T wire. 

Thermosetting Plastic 
Material that can be softened a single time by heat. After one heat application, the substance becomes a heat resistant material. 

Thermosetting Resin 
Material that can be softened a single time by heat. After one heat application, the substance becomes a heat resistant material. 

Thermosiphon Effect 
Movement of fluid induced by heating. The fluid expands as it is heated and is forced through the system. 

Thermostat
A switch using a bimetallic strip that reacts to changes in temperature. Thermostats are used to regulate heating and cooling systems by switching the heating/cooling on or off as temperature changes. 

Thermowelding
The connection of plastic fittings by melting at the joints. 

Thermostatic Control 
Device used to regulate the point at which the burner for the water heater is activated. 

Thickened-end Slab 
A concrete slab that is thicker at the perimeter to support the walls of the structure. 

Thickness Gauge 
Also called a feeler gauge, this series of precision thickness, metal blades is used for measuring small gaps, with each blade being marked with the thickness in thousandths of an inch. 

Thin Market 
Market in which there are comparatively few bids to buy or offers to sell real estate. The term relates to a single investment or to a particular investment market, such as the real estate market. In a thin market, buying or selling a few homes can impact disproportionately. Price volatility is generally wider than liquid markets. 

T-Hinge 
A hinge shaped like a T, typically used for decorative purposes in period houses. 

Thinner 
A petroleum-based solvent used for thinning and removing oil-based paint. 

Third Party 
One who is not directly involved in a transaction or contract but may be involved or affected by it. 

Third-Party Origination 
Transaction where the lender has another institution originate all or part of a mortgage. 

Thread Gauge
A gauge used form checking the accuracy of a threaded component. 

Threads
Spiral grooves cut into a fastener or component to facilitate connection with other grooved items. Nuts and bolts are examples of threaded fasteners. 

Three-corner File 
A file with a triangular blade, used for work in inaccessible spots. 

Three-prong Plug 
An electrical plug with two flat prongs and one round one (used as a ground). Typically used with high current devices such as refrigerators. 

Three-way Switch 
Switch device, which uses two switches to allow an outlet or light to be activated from either switch. 

Three-way Valve 
A valve with a single inlet and two outlets. The valve can be set so that either outlet is open. 

Three-wire System 
An electrical system using two hot leads and one ground. Typically used for 240-volt service. 

Threshold
1. The entrance to a building or room. 
2. A wood or metal strip over an exterior door sill. 
3. A limit. 

Threshold Limit Value 
Levels of airborne concentrations of chemicals, used to determine safety limits. 

Thrift Institution 
Depository institution, such as mutual savings banks. If organized as mutual associations, depositors are shareholders. They provide mortgages. 

Throat
A constriction in a passage. 

Through Drying 
Total drying of a painted surface. 

Throw
The extent of an area illuminated by a given light fixture. 

Thrust
The application of force in one direction. 

Thrust Block 
Concrete reinforcement poured under a pipe where a direction change occurs. The block supports the pipe and prevents it from moving under the force of the moving liquid inside. 

Thumb Screw 
A screw with a large head, designed to be tightened by hand.