The definition of megahertz

A million cycles per second, abbreviated MHz. This is often used misleadingly to indicate processor speed, because while one might expect that a higher number would indicate a faster processor, that logic only holds true within a given type of processors as different types of processors are capable of doing different amounts of work within a cycle. For a current example, either a 200 MHz PowerPC or a 270 MHz SPARC will outperform a 300 MHz Pentium.