Glossary
Code without tests that earns money. The definition by Michael Feathers is useful because it names the real problem: missing safety, not age.
Age alone does not make code legacy. A ten year old system with good test coverage can be changed safely. A two year old one without tests cannot.
The marker is therefore fear of change. It is rational as long as nobody can show that a change breaks nothing.
How you notice it
Frequently asked
With a seam, not with cleaning up. Find a place where behaviour can be checked from the outside and write a test that pins down today behaviour. From there every further change can be secured without understanding the whole system.