Glue Code

Definitions

Code that exists solely to connect two systems, libraries, or components that weren't designed to work together. Nobody wants to write it, everyone depends on it, and it's never in the architecture diagrams.
In plain English: Code that exists only to connect two things that weren't designed to work together — like an adapter plug between different outlet types. Nobody loves writing it, but somebody has to.
Glue code often becomes the most fragile part of a system because it depends on the interfaces of multiple components — when either side changes, the glue breaks. It's also rarely tested thoroughly because it's seen as 'just wiring,' not 'real logic.'
Example: '80% of our bugs are in the glue code between services. Nobody owns it, nobody tests it, but everything depends on it.'
Source: fragility / ownership problem