A second source is a semiconductor fab or manufacturer that holds the rights to build and sell a drop-in replacement for a specific part. Buyers and military contractors often require a second source before designing a chip into a system to guarantee supply chain stability if the original creator goes out of business or faces production delays.
Heard on the job
Purchasing won't approve the new microcontroller unless we can find a reliable second source for it.
Insider tip
A true second source has official masks or licensing from the original designer. Parts that just happen to share a pinout are considered functional equivalents, not second sources.