Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are generally considered in classical two-body systems, where a Kepler orbit is used (derived from Newton's laws of motion and Newton's law of universal gravitation). There are many different ways to mathematically describe the same orbit, but certain schemes each consisting of a set of six parameters are commonly used in astronomy and orbital mechanics.
A real orbit (and its elements) changes over time due to gravitational perturbations by other objects and the effects of relativity. A Keplerian orbit is merely an idealized, mathematical approximation at a particular time.
Read more about Orbital Elements: Keplerian Elements, Required Parameters, Alternative Parametrizations, Orbit Prediction, Perturbations and Elemental Variance, Two-line Elements
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“Our institutions have a potent digestion, and may in time convert and assimilate to good all elements thrown in, however originally alien.”
—Herman Melville (1819–1891)