Demand is measured in units of power, most notably in KW (for electrical use). It is uncommon for a residence, but for commercial buildings, Utility companies may charge for the peak demand use.
DX stands for direct expansion. In HVAC, DX refers to an air conditioning unit that has heat exchange directly between a [hermetically sealed] refrigerant side, and the supply air
An airside economizer is a device that brings in more outside air to provide additional cooling. It is often based on the drybulb temperature of the outdoor air. Another common control strategy is comparitive dry bulb - where the building controls determine if the outdoor air has a lower dry bulb than the return air (or has lower enthalpy), additional outdoor air enters the supply air. Airside economizers typically allow a maximum of 100% of the supply airflow to be outside air. Airside economizers are required for most buildings based on ASHRAE std 90.1
A waterside economizer is a heat exchange device that sends condenser return water to a exchange heat with cooling tower (or a dry cooler) water, whicht is cooled by ambient air. The return water is used to help a chiller operate more efficiently, or in some cases to meet the entire cooling load.
In HVAC, electric resistance refers to heating that is done by running electricity through a material that gets hot when electric current passes through it (due to electric resistance).
Evaporative cooling is the process of cooling by the evaporation of water, which increases the humidity, but decreases the dry bulb temperature. There are several types of evaporative cooling. Typically, evaporative cooling refers to direct evaporative cooling, often referred to as a swamp cooler. In a swamp cooler, warm (dry) air is forced through a wet medium, and is cooled as it evaporates water. The exiting air has a lower dry bulb temperature, but a higher dew point. This type of cooling is common in the American Southwest
In dry climates, water may be sprayed via a mist onto an air cooled condenser's exterior, cooling it, and allowing condenser to operate at a lower temperature, which increases the overall efficiency of the compressor