True Level 
A distance between two points which are at the same height, also called Dead Level. If the area between the two points are not even and at the same level as the end points then additional topdressing can be added to raise up any depressions to make even.
UK Soil Textural Triangle
Unavailable Water
Water which is not available for use by a plant. This is usually Gravitational Water and also water held in the soil at Permanent Wilting Point.
Underdrainage
An outdated term used to describe a drainage system installed in a field. This would be in contrast to surface drainage which would be surface water running off a field, especially into a drainage ditch at the edge of the field.
Underground Service
A reference to a utility service such as an electric cable, water pipe, gas pipe, media cable, drainage pipe, irrigation pipe, or similar. All of these might be located below a surface. When carrying out construction work, deep aeration or similar types of work it is essential to know the location of all underground services in the work area.
Undersoil Heating
Typically a network of water pipes buried beneath a turfgrass surface. or electrical cables A heated rootzone can reduce the chance of lost matches from heavy frosts and ice. These are costly to install so would only be considered where professional league pitches are concerned. The first undersoil heating at a football club pitch was over 50 years ago at Everton FC.
USDA Soil Textural Triangle
Vermiculite
A 2:1 layer clay mineral. These are typically found in sediments.
Volumetric Water Content
The volume, expressed as a percentage, of the soil which contains water.
Warm Soil
A free draining sandy soil which can warm up quicker than a cold soil, especially in Springtime. With less water being retained by these soils after natural drainage the water content does not slow the warming of the soil as much as heavier, more water retentive soils.