Essential dignity, in astrology, refers to the relative strength or weakness of a planet or point's zodiac position by sign and degree, or its essence—what the pre-eminent 17th-century astrologer William Lilly called "the strength, fortitude or debility of the Planets [or] significators." In other words, essential dignity seeks to view the strengths of a planet or point as though it were isolated from other factors in the sky of the natal chart.
By comparison, accidental dignity indicates how much strength a planet or point derives from its position in a natal chart, such as its relation to the other factors in the chart: for example, its proximity to other planets, or to the four angles of the chart, or to stars, as well as the aspects (or symmetrical angular connections) it forms with other planets or points in the chart.
For example, the essential dignity of Mars, if located at 27 degrees of Capricorn would take into account the fact that Mars is exalted in Capricorn, and also that it is the bounds ruler of the 27th degree of Capricorn and also the face ruler of the 27th degree of Capricorn. This is a considerably dignified Mars.
If that Mars were located in the twelfth house of a natal chart Mars's accidental dignity would be poor, since it would be located in a weak or malevolent cadent house. Were Mars to also be squared to a malevolent planet, such as Saturn, and would be receiving a dexter square aspect from malefic Saturn, ruler of Capricorn, this would further hinder Mars's strength and ability to operate benevolently. These accidental dignity factors would tend to weaken a Mars which is otherwise strong in essential dignity.