Original: Copyright infringement is the unauthorized or prohibited use of works under copyright, infringing the copyright holder's exclusive rights, such as the right to reproduce or perform the copyrighted work, or to make derivative works.
Simplified: Copyright infringement (or copyright violation) is the use of material which is covered by copyright law, in a way that violates one of the original copyright owner's exclusive rights, such as the right to reproduce or perform the copyrighted work, or to make derivative works that build upon it.