The term 'relativity' is usually associated with Einstein's theory of relativity and if you ask someone casually 'what is relativity?', the answer would normally be that it is a theory that Einstein dreamt up. This is not true. Galileo Galilei (1632) Galileo Galilei first described 'the principle of relativity' in 1632 in his 'Dialogue Concerning the Two Chief World Systems'. Galileo used the example of a ship traveling at constant speed, without rocking, on a smooth sea; any observer doing motion experiments below the deck would not be able to tell whether the ship was moving or stationary. This lead Galileo to state that the laws of physics are the same in any system that is moving at a constant speed in a straight line, regardless of its particular speed or direction. Hence, there is no absolute motion and no absolute rest. This was a far reaching and quite visionary insight by Galileo, more than two centuries ahead of his time. The principle is sometimes called 'Galilean relativity'. Sir Isaac Newton (1687) Galileo's work provided the basic framework for Sir Isaac Newton's three laws of motion, published in his monumental 1687 work, now known as the 'Principia'. Hence Galileo's relativity principle is also sometimes, perhaps wrongly, called 'Newtonian relativity'". Newton's three laws of motion can be briefly stated as: (i) An object at rest tends to stay at rest and an object in uniform motion tends to stay in uniform motion, unless acted upon by an external force. (ii) The acceleration of an object is directly proportional to the magnitude of the force acting on it and inversely proportional to its mass. (iii) For every action there is an equal and opposite reaction. Significant as these laws are, in a way they regressed backwards when compared to Galilean relativity. Newton stated his laws to hold in 'absolute space', meaning that there must be absolute motion and absolute rest. This was partially caused by the view of the time that in empty space light moves at a constant speed through an invisible medium called the 'aether'. Galileo did not believe in absolute motion, but like Newton, he believed in a universal time that is the same for every uniformly moving reference system (called an 'inertial frame'), regardless of its velocity. Albert Einstein (1905) On the matter of universal time, Einstein differed from both Galileo and Newton. In his groundbreaking 1905 paper, 'On the Electrodynamics of Moving Bodies', he abandoned universal time and postulated that every inertial frame has its own unique time. Einstein restated Galileo's principle of relativity, where the laws of physics are the same in every inertial frame, regardless of its particular speed or direction. Einstein's 1905 insight extended Galilean relativity in one crucial aspect: the speed of light in free space (symbol c) is the same in every inertial frame of reference, irrespective of its movement. Incidentally, Galileo also thought that the speed of light is the same in every inertial frame, but that's only because he reasoned that light's speed was near infinitely large. If you take Einstein's equations of special relativity and set the speed of light to infinity, you end up with Galileo's principle of relativity. So in a way, Galileo and Einstein were closer in their principles of motion than what Newton and Einstein were. Newton (like Einstein) knew that light had a finite, measurable speed. Newton did however (wrongly) hold that the speed of light is only constant in absolute space, meaning that it must have different values relative to moving observers. If Newton has been right, the speed of light would have been different in different directions inside a moving inertial frame. This has been ruled out by experiments. Einstein postulated that the relative nature of motion and of time dictates that all inertial frames will measure the same constant speed of light in all directions. Experiments have confirmed this view. Einstein's principles of special relativity can be briefly summarized as follows: i) There is no observable absolute space or absolute motion. ii) There is no universal time. Every inertial frame has its own time. iii) The laws of physics are the same in every inertial frame. iv) The measured speed of light is independent of relative motion. These four principles imply that time and distance measurements differ in various inertial frames. In general, if two observers are moving relative to one another, they will disagree on the distance between two objects and also on the time that it takes light to travel that distance. This is the essence of Einstein's special theory of relativity. Gravity: Newton (1687) - Einstein (1916) Einstein's general theory of relativity of 1916 extended his special theory to include gravity, essentially replacing Newton's theory of universal gravitation, as described in his 'Principia' of 1687. When the gravitational field is weak and the speeds are very small compared to the speed of light, Newton's theory of gravity is accurate enough for all practical purposes. Einstein's theory of relativity only becomes a requirement when speeds are significant fractions of the speed of light and/or the gravitational fields are thousands of times stronger than what we experience here on Earth. Such conditions are observed near neutron stars and black holes, but that's another story.