A condition representing planetary contentment because the planet is suitably positioned according to sign and sect. The term is usually applied when the planetary nature agrees with its the hemisphere placement (by itself termed halb, 'half') and the gender of its sign. For example, when a masculine, diurnal planet is positioned in the same hemisphere as the Sun (above or under the Earth) and is also in a masculine sign. Likewise; or when a feminine, nocturnal planet is placed in the opposite hemisphere to that of the Sun and is in a feminine sign.
The word has slightly variant definitions according to different traditional authors because it derives from an Arabic term which simply describes the placement of the planet as a suitable one. For example, the Arabic astrologer Al-Biruni* tells us that a masculine planet is more dignified in a masculine sign, so he says it is in its hayyiz (or hayz); meaning its 'natural place' or 'preferred position'. The placement of a planet in an unsuitable place - such as a masculine planet in a feminine sign, is considered weakening and termed 'contrariety of hayz' or contention.
In his Opusculum Astrologicum (1539) Johannes Schoener tells us that the condition is also known as 'similitude'. He says:
Hayz, or planetary similitude, is when a diurnal planet is above the earth in the day, under the earth at night, and a nocturnal planet is under the earth by day, and above the earth at night.
Or, again, when a masculine planet is in a masculine sign and quarter and is oriental, but a feminine planet is in a feminine sign and quarter and is occidental.
This method sometimes comes to three times in which a planet can be in its own hayz; first, agreement to the quality of time [by being] above or under the earth; second, agreement with the masculinity or femininity of the sign; third, agreement with the masculine or feminine quadrants, which is called by others conformity by quadrant.
Observe, however, that Mercury is sometimes diurnal, sometimes nocturnal, now masculine, then feminine, according to its configuration with planets, or according to the nature of the sign in which it is found, if it is not conjunct or configured by another [planet], which you should note well. (II, XIX).
In his table of dignity scores, Schoener includes 3 extra points of fortitude for a planet in hayz, and deducts 2 points of strength for a planet in 'contraiety of hayz'. Lilly's definition (CA, p.113) reads:
Hayz is when a masculine and diurnal Planet is in the day time above the earth and in a Masculine sign, and so when a feminine, nocturnal planet in the night is in a feminine Sign and under the earth: in questions it usually shows the content of the querent at the time of the question, when his significator is so found.
*The Book of Instruction in the Elements of the Art of Astrology, (written 1029, translated 1934 by R. Ramsay Wright) ch. 496 & 497, p.308.