
Base load

The baseload (also base load) on a grid is the minimum level of demand on an electrical grid over a span of time, for example, one week. This demand can be met by unvarying power plants, dispatchable generation, or by a collection of smaller intermittent energy sources, depending on which approach has the best mix of low cost, availability and high reliability in any particular market. The remainder of demand, varying throughout a day, is met by dispatchable generation which can be turned up or down quickly, such as load following power plants, peaking power plants, or energy storage.
Power plants that do not change their power output quickly, such as large coal or nuclear plants, are generally called baseload power plants. Historically, most or all of baseload demand was met with baseload power plants, but this is becoming less common due to increasing levels of renewables and advances in flexible generation.
Description.
Grid operators take long and short term bids to provide electricity over various time periods and balance supply and demand continuously. The detailed adjustments are known as the unit commitment problem in electrical power production.
While historically large power grids used unvarying power plants to meet the base load, there is no specific technical requirement for this to be so. The base load can equally well be met by the appropriate quantity of intermittent power sources and dispatchable generation.
Unvarying power plants can be coal, nuclear, combined cycle plants, which may take several days to start up and shut down, hydroelectric, geothermal, biogas, biomass, solar thermal with storage and ocean thermal energy conversion. 
The desirable attribute of dispatchability applies to some gas plants, wind (through blade pitch) and hydroelectricity. Grid operators also use "curtailment" to shut plants out of the grid when their energy is not needed.
There are 195 gigawatts of grid storage installed world-wide; 94% is pumped-storage hydroelectricity; 2% is in batteries. 
Pumped storage uses cheap power at times of low demand, usually night, to pump water from a lower reservoir to an upper reservoir, then lets it drop back through turbines during peak demand times, usually in the day. Availability of solar power in peak hours of the day can reduce the need for storage. The biggest storage facility in the world is on the Virginia-West Virginia border, with 50% more capacity than the Hoover Dam.
Economics.
Grid operators solicit bids to find the cheapest sources of electricity over short and long term buying periods.
Nuclear and coal plants have very high fixed costs, high plant load factor but very low marginal costs, though not as low as solar, wind, and hydroelectric. On the other hand, peak load generators, such as natural gas, have low fixed costs, low plant load factor and high marginal costs.
Coal and nuclear power plants do not change production to match power consumption demands since it is more economical to operate them at constant production levels, and not all power plants are designed for it. However, some nuclear power stations, such as those in France, are physically capable of being used as load following power plants and do alter their output, to some degree, to help meet varying demands. 
Some combined-cycle plants usually fuelled by gas, can provide baseload power, as well as being able to be cost-effectively cycled up and down to match more rapid fluctuations in consumption. 
Different plants and technologies may have differing capacities to increase or decrease output on demand: nuclear plants are generally run at close to maximum output continuously (apart from maintenance, refueling and periodic refurbishment), while coal-fired plants may be cycled over the course of a day to meet demand. Plants with multiple generating units may be used as a group to improve the "fit" with demand, by turning units on and off.
According to National Grid plc chief executive officer Steve Holliday in 2015, baseload is "outdated", as microgrids would become the primary means of production, and large powerplants relegated to supply the remainder.