Definitions

The day has several definitions.

International System of Units (SI)

A day contains 86,400 SI seconds. Each second is currently defined as

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

Astronomy

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

All planets have a rotation period relative to the fixed stars, commonly called their sidereal rotation periods.Allen's astrophysical quantities page 296. For Earth this is 23hours and 56 minutes and 50 seconds. Earth also has a misnamed sidereal day, its rotation relative to its own precessing or moving mean vernal equinox, which is 8.4 ms smaller than its rotation period relative to the fixed stars. The rotation periods of the gas giants, which have no solid surface, are usually assumed to be the rotation of the magnetic fields residing in their deep interiors unless stated otherwise. Jupiter and Saturn also have specified equatorial and high latitude rotation periods for their cloud tops.

Colloquial

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

• The period of light when the Sun is above the local horizon (i.e., the period from sunrise to sunset), opposed to night. See Daytime (astronomy).
• 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 period from 06:00 to 18:00 or 21:00 or some other fixed clock period overlapping or set off from other periods such as "morning", "evening", or "night".
• The mostly regular interval of one awaking, usually in the morning (personal day).

Introduction

The word day is used for several different units of time based on the rotation of the Earth around its axis. The most important one follows the apparent motion of the Sun across the sky (solar day; see solar time). The reason for this apparent motion is the rotation of the Earth around its axis, as well as the revolution of the Earth in its orbit around the Sun.

A day, as opposed to night, is commonly defined as the period during which sunlight directly reaches the ground, assuming that there are no local obstacles. Two effects make days on average longer than nights. The Sun is not a point, but has an apparent size of about 32 minutes of arc. Additionally, the atmosphere refracts sunlight in such a way that some of it reaches the ground even when the Sun is below the horizon by about 34 minutes of arc. So the first light reaches the ground when the centre of the Sun is still below the horizon by about 50 minutes of arc. The difference in time depends on the angle at which the Sun rises and sets (itself a function of latitude), but amounts to almost seven minutes at least.