A day (symbol d) is a unit of time equivalent to approximately 24 hours. It is not an SI unit but it is accepted for use with SI. The SI unit of time is the second.

The word 'day' can also refer to the (roughly) half of the day that is not night, also known as 'daytime'. Both refer to a length of time. Within these meanings, several definitions can be distinguished. 'Day' may also refer to a day of the week or to a calendar date, as in answer to the question "On which day?".

The term comes from the Old English dæg, with similar terms common in all other Indo-European languages, such as Tag in German, dies in Latin, dydd in Welsh or dive in Sanskrit.

International System of Units (SI)

A day is defined as 86,400 seconds.

A day on the UTC time scale can include a negative or positive leap second, and can therefore have a length of 86,399 or 86,401 seconds.

The International Bureau of Weights and Measures (BIPM) currently defines a second as

… the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium 133 atom.

In the 19th century it had also been suggested to make a decimal fraction (frac: 10,000 or frac: 100,000) of an astronomic day the base unit of time. This was an afterglow of decimal time and calendar, which had been given up already.

Astronomy

A day of exactly 86,400 SI seconds is the fundamental unit of time in astronomy.

For a given planet, there are two types of day defined in astronomy:

• 1 apparent sidereal day - a single rotation of a planet with respect to the distant stars (for Earth it is 23.934 hours);
• 1 solar day - a single rotation of a planet with respect to its star.

In astronomy, the sidereal day is also used; it is about 3 minutes 56 seconds shorter than the solar day, and close to the actual rotation period of the Earth, as opposed to the Sun's apparent motion. In fact, the Earth spins 366 times about its axis during a 365-day year, because the Earth's revolution about the Sun removes one apparent turn of the Sun about the Earth.

Colloquial

The word refers to various relatedly defined ideas, including the following:

• 24 hours (exactly)
• the period of light when the Sun is above the local horizon (i.e., the time period from sunrise to sunset);
• the full day covering a dark and a light period, beginning from the beginning of the dark period or from a point near the middle of the dark period;
• a full dark and light period, sometimes called a nychthemeron in English, from the Greek for night-day;
• the time period from 6:00 AM to 6:00 PM or 9:00 PM or some other fixed clock period overlapping or set off from other time periods such as "morning", "evening", or "night".