Alarm Clock – A clock designed to wake a person at a specific time. Some water clocks and mechanical clocks were capable of chiming at a fixed time every day, but the first user-settable alarm clocks date back to 15th Century Europe, and the traditional mechanical wind-up alarm clock, settable for any time and small enough to use on a bedside table, appeared in Europe and the USA in the mid to late 19th Century.

Atomic Clock – An extremely accurate timekeeping device or clock, regulated by the natural vibration frequency of atoms or molecules of substances like caesium, ammonia or hydrogen. Atomic clocks are the most accurate clocks we have, and are now used as primary standards for international time distribution services.

Calendar – A system of organizing days for social, religious, commercial or administrative purposes, typically by dividing time up into units of days, weeks, months and years. There are various different kinds of calendar, the most common being lunar calendars, solar calendars and luni-solar calendars.

Calendar Date – A particular day represented within a calendar system, the usual meaning of the word date.

Calendar Year – An approximation of a solar year in a given calendar system, the usual meaning of the word year.

Chronology – The science of arranging events in their order of occurrence in time, and ascertaining the dates and historical order of past events. It can be considered a part of periodization and of the study of history.

Civil Time – The statutory time scale designated by civilian authorities to indicate the official local time. This is generally Standard Time in a particular time zone, which is set as a fixed offset from Coordinated Universal Time (UTC) or Greenwich Mean Time (GMT), possibly adjusted by Daylight Saving Time (DST) during part of the year.

Date – This usually refers to a calendar date, a particular day represented within a calendar system. A particular date is usually specified according to unit divisions like days, months and years and a starting reference point or fiduciary epoch (e.g. 3rd of May 2013CE).

Duration – Informally, duration is the measure of continuance of any object or event within time. In philosophy, it refers more specifically to Henri Bergson’s theory of subjective and ineffable time that can only be grasped through a simple intuition of the imagination.

Electric Clock – A clock powered by electricity rather than a hanging weight or a spring), although it usually refers to the electrically-powered mechanical clocks that were used before electronic clocks replaced them in the 1980s. They were first introduced in the 1840s, and became the most widely used type of clock by the 1930s.

Event – An object, physical situation or occurrence in time. Or, from the point of view of relativistic physics, a particular location in space-time (i.e. a point in space at an instant in time). Space-time as a whole is a collection of an infinite number of events.

Geological Time Scale – The time scale used in periodization that is appropriate to deep time, outside the human time scale, covering periods and events in the history of the Earth of the magnitude of many millions or even billions of years.

Hour – A unit of measurement of time, comprising 60 minutes or 3,600 seconds. 24 hours make up a day. Although it is not itself a standard unit defined by the International System of Units (SI), the hour is a unit accepted for use with SI.

Interval – The duration of time between two events, or the period of time marked off by two events.

Leap Year – A year that requires the addition of a leap day in order to synchronize a particular calendar with the seasons. This is necessary because a solar year in not an exact number of days (approximately 365.242 days), and without this intercalation the discrepancy would accumulate more and more each year until the calendar was out of sync with the seasons.

Minute – A unit of measurement of time, comprising 60 seconds. 60 minutes make up one hour. Although it is not itself a standard unit defined by the International System of Units (SI), the minute is a unit accepted for use with SI.

Month – A unit of measurement of time, roughly based on the period of a complete revolution of the Moon around the Earth (lunar month). There may be anywhere from 28 to 31 days in a month, depending on the kind of calendar used, and twelve or thirteen months in a year.

Second – A unit of measurement of time, and the base unit of time in the International System of Units (SI). 60 seconds make up one minute.

Solar Time – Time measurement based on the rotation of the Earth on its axis, using the Sun’s apparent motion across the sky to measure the duration of a day. Because the Earth’s orbit is an ellipse rather than a circle, its speed around the Sun is not constant.

Time – A dimension in which events can be ordered from the past through the present and into the future, and also the measure of durations of events and the intervals between them. Time can be seen as the indefinite continued progress of existence and events in the past, present, and future, regarded as a whole.

Timestamp – A sequence of characters or encoded information identifying when a certain event occurred, usually giving a date and a time of day. An example is a postmark on a letter, but these days it is more likely to refer to digital date and time information embedded in computer files or digital pictures.

Time Zone – A region that has a uniform civil time for legal, commercial and social purposes, i.e. the usual clock time that most people use in daily life. Standard Time divides the world into 24 time zones, each one covering 15 degrees of longitude (although, for reasons of practicality.