1. n. 
A subset of the more general term "directional drilling," used where the departure of the wellbore from vertical exceeds about 80°. Note that some horizontal wells are designed such that after reaching true 90° horizontal, the wellbore may actually start drilling upward. In such cases, the angle past 90° is continued, as in 95°, rather than reporting it as deviation from vertical, which would then be 85°. Because a horizontal well typically penetrates a greater length of the reservoir, it can offer significant production improvement over a vertical well.
2. n. 
The intentional deviation of a wellbore from the path it would naturally take to a horizontal trajectory. Horizontal lateral sections can be designed to intersect natural fractures or simply to contact more of the productive formation. Horizontal drilling is accomplished through the use of whipstocks, bottomhole assembly (BHA) configurations, instruments to measure the path of the wellbore in three-dimensional space, data links to communicate measurements taken downhole to the surface, mud motors and special BHA components, including rotary steerable systems and drill bits. While many techniques can accomplish this, the general concept is simple: Direct the bit in the direction that one wants to drill. By placing a bend near the bit in a downhole steerable mud motor, the bend points the bit in a direction different from the axis of the wellbore when the entire drillstring is not rotating. By pumping mud through the mud motor, the bit turns while the drillstring does not rotate, allowing the bit to drill in the direction it points. When a particular wellbore direction is achieved, that direction may be maintained by rotating the entire drillstring (including the bent section) such that the bit does not drill in a single direction off the wellbore axis. Instead, the bit sweeps around and its net direction coincides with the existing wellbore. Rotary steerable tools allow steering while rotating, usually with higher rates of penetration and ultimately smoother boreholes. Horizontal drilling is common in shale reservoirs because it allows drillers to place the borehole in contact with the most productive reservoir rock.