Legacy is an interesting term. In general usage, it refers to something of value that has been passed down through the generations. When talking about legacy in technology we have something that was passed down to us, but usually it is neither wanted nor desired.
One of the hardest parts of working with legacy systems is that the infrastructure is often poorly understood and usually neglected, despite running the most critical parts of an organization. For example, COBOL (the computer programming language whose heydays ended in the early eighties) still runs most of the financial transactions in the world and IBM mainframes are still the beating hearts and guiding hands for many organizations.