What does the error "Use of undefined constant CRAFT_BUILD - assumed 'CRAFT_BUILD'?" mean? Accessing my /admin of a craft site and i get this error.

Internal Server Error

Use of undefined constant CRAFT_BUILD - assumed 'CRAFT_BUILD'


Never happened before today, I'm running PHP v5.6.30.

Thanks
Had a plugin installed which had been using CRAFT_BUILD and as its been deprecated it was throwing the error, problem was could access admin to update plugin.

Manually, entered CRAFT_BUILD and other deprecated info into craft/app/Info.php and it let me login to fix.