Legacy system
Old software a business still depends on and cannot easily replace.
A legacy system is software, and often the hardware under it, that is old but still doing important work. Think of a bank's core accounts system written decades ago, an airline's booking engine or the program that runs a government's tax records. Many are written in old languages like COBOL and run on mainframes. Nobody would build them that way today, but they work, and the business runs on them.
The trouble is that change gets harder every year. The people who wrote the code have retired, the documentation is thin, developers who know the language are scarce and expensive, and the system may not connect easily to modern apps or the cloud. Every new feature turns into a careful operation on code nobody fully understands.
Replacing it is risky too: a full rewrite can take years and break things the old system handled quietly. So many companies modernize step by step, wrapping the old system in an API so new apps can talk to it, then moving pieces out one at a time. "Legacy" is not an insult; it usually means the software was useful enough to outlive its era.
Keep in mind
- A legacy system is old but still critical to the business.
- Banks, airlines and governments run many of them, often in COBOL on mainframes.
- Changing them is slow, risky and expensive, and experts are scarce.
- Most organizations modernize gradually instead of rewriting everything at once.
