Sir Isaac Newton

Sir Isaac Newton defined inertia in Definition 3 of his Philosophiæ Naturalis Principia Mathematica, which states:
        The vis insita, or innate force of matter, is a power of resisting by which every body, as much as in it lies, endeavours to preserve its present state, whether it be of  rest or of moving uniformly forward in a straight line.

Below is a short video about Sir Isaac Newton and his contributions to physics:

Isaac Newton, Mathematical Principles of Natural Philosophy translated into English by Andrew Motte, First American Edition, New York, 1846, page 72.
http://www.youtube.com/watch?v=iH48Lc7wq0U


Edited:  Ian