You may find a variety of slightly different definitions of baseload; which tells you that it isn’t really a technical term with a precise physical definition. Broadly, it’s a term used to describe the minimum amount of power (MW, GW etc.) that a power system needs to deliver (to meet the minimum level of demand) over a period of time, such as a twenty-four hour period.
One view is that the delivery of this minimum level of output is the job of “baseload power plants”: low-fuel-cost and often inflexible generators operating at high capacity factors (i.e. operating most of the time, including through the night when power demand is at a minimum).
A different view is that, in future, the concept of baseload power plants operating on a steady basis becomes redundant in a world where variable renewable power sources such as wind or solar are given dispatch priority (i.e. used first). In this case more flexible generating options are needed to meet whatever level of demand requirement can’t be met by variable resources, turning up and down as and when required to fill the gap between variable supply and total power demand.