What is the clear definition of evolution ?

As defined by the urban dictionary, evolution is the descent with modification from preexisting species : cumulative inherited change in a population of organisms through time leading to the appearance of new forms : the process by which new species or populations of living things develop from preexisting forms through successive generations

Basic Definition of Evolution: That everything in the Universe, including Life on this planet, evolves or changes over time, particularly, over Geologic or Cosmological time of millions and billions of years, when it becomes very evident. It evolves ALL the time, except it may not be perceived by the human eye.