Any Software, Hardware, installation or Protocol which has been superseeded. Generally the older an organisation, the more legacy systems they use, becuase the cost of upgrading large organisations tends to be very high.
For example, ISA might be referred to as a legacy Bus standard and any Linux Kernel older than the current stable branch is a Legacy kernel. Working with legacy systems is frustrating becuase they have known problems which are trivially fixable (by upgrading), but there are non-techincal constraints preventing them from being fixed.
Also the name of a linux distribution... the older version of Fedora that are not "officially" supported - see FedoraLegacy