Newton's First Law of Motion:

Every body perseveres in its state of being at rest or of moving uniformly straight forward, except in so far as it is compelled to change its state by forces impressed.

(Translated from the original Latin by I.E. Cohen and A. Whitman.)

For an explanation of Newton's first law and for the role of Newton's first law in the definition of inertial frames of reference, see Force/Newton's First Law/Explain It.

Newton's first law is also called the "Law of Inertia".