A 'history painting' is one which has a serious narrative, or includes exemplars of actions which are intended to have didactic overtones. In this sense the word history relates to the Italian istoria, meaning narrative or story (and not the accurate or documentary description of actual events).
History paintings are often large in scale. Their subjects can be taken from the Bible, from mythology or other forms of secular literature, from historical events; or they can be allegories. Noble themes are seen as being particularly worthy of depiction.
History painting was viewed as the most important of the genres from about the 16th century, and the climax of an academic painter's training. It was the equivalent of Epic or Tragedy in literature.