Relative hour (Hebrew singular: shaʿah zǝmanit / שעה זמנית; plural: shaʿot - zǝmaniyot / שעות זמניות), sometimes called halachic hour, temporal hour, seasonal hour and variable hour, is a term used in rabbinic Jewish law that assigns 12 hours to each day and 12 hours to each night, all throughout the year. A relative hour has no fixed length in absolute time, but changes with the length of daylight each day - depending on summer (when the days are long and the nights are short), and in winter (when the days are short and the nights are long). Even so, in all seasons a day is always divided into 12 hours, and a night is always divided into 12 hours, which invariably makes for a longer hour or a shorter hour. At Mediterranean latitude, one hour can be about 45 minutes at the winter solstice, and 75 minutes at summer solstice. All of the hours mentioned by the Sages in either the Mishnah or Talmud, or in other rabbinic writings, refer strictly to relative hours.

Another feature of this ancient practice is that, unlike the standard modern 12-hour clock that assigns 12 o'clock pm for noon time, in the ancient Jewish tradition noon time was always the sixth hour of the day, whereas the first hour began with the break of dawn according to many Halachic authorities, and with sunrise according to others. Midnight (12:00 am local official clock time) was also the sixth hour of the night, which, depending on summer or winter, can come before or after 12:00 am local official clock time, whereas the first hour of the night always begins after sunset, when the first three stars appeared in the night sky.

During the Spring (באחד בתקופת ניסן‎) and Autumnal (באחד בתקופת תשרי‎) equinox (around 20 March and 23 September), the length of a day and night are equal. However, even during the summer solstice and winter solstice when the length of the day and the length of the night are at their greatest disparity, both day and night are always divided into 12 hours.

Temporal hours were common in many cultures. A division of day and night into twelve hours each was first recorded in Ancient Egypt. A similar division of day and night was later made in the Mediterranean basin from about Classical Greek Antiquity into twelve temporal hours each (Ancient Greek: ὥραι καιρικαί, romanized: horai kairikai).

In Western culture they were adopted from Roman timekeeping and were adopted in the European Medieval era. They had particular relevance in the fixed daily schedule of the monastic orders. This division of time allowed the work of the day -such as eating, praying, or working -to always be performed at the same (temporal) hour, regardless of season (Prayer of the Hours).

The prevailing opinion is that each day begins at the rise of dawn (Heb. עלות השחר‎), which is about 72 minutes before sunrise, yet, for practical reasons in some biblically related commandments, some scholars begin counting the hours of the day from sunrise (Heb. הנץ החמה‎), such as for the recital of Shema' which, as a first resort, must be recited when a person rises from his sleep in the morning, a time that is traditionally linked with sunrise, and continuing thereafter until the beginning of the 4th hour of the day, or, for example, when burning leaven on the 14th day of the lunar month Nisan, which must be burnt in the 6th hour of the day when counting from sunrise. At this time, the sun is nearly at its apex.

The commencement of nightfall is not as divisive in Jewish law:

Rabbi Pinchas said in the name of Rabbi Abba bar Pappa: One star is certainly day; two [stars] is a doubtful case; three [stars] is certainly night.