In power generation, the term demand is most often used to describe the rate of electricity usage (or expected usage) at a particular point in time. Thus “peak demand” would refer to the maximum (e.g. 50GW at 5pm), while “baseload demand” would refer to the minimum (e.g. 25GW at 3am).
So a “demand curve” would typically plot power against time, to illustrate changing levels of electricity usage over different timescales (e.g. night vs. day or summer vs. winter). Note that this is different from the concept of a demand curve in economics, which plots the price of something against market size at each different price.
However you will sometimes see the term demand used in energy terms – where really the term “consumption” would be preferable – to describe the aggregated usage of energy over a period of time:, e.g.: “annual household electricity demand is 5,000kWh”.
To further add to the confusion, you’ll find the term “load” used interchangeable with demand, when talking in power usage terms (i.e. “peak load = 50GW”).