ctags
A tool that scans source code and writes an index of where every function, class and variable is defined. The editor reads the file and can jump to the definition — a technique from the 1980s.
The format is a sorted text file: symbol name, file name and a search pattern. In Vim you press Ctrl-] on a name and land in the right place. It's crude compared with a language server — ctags doesn't understand scope, types or overloading, so three different init methods yield three equally plausible matches — but it's also language-agnostic, needs no compilation and works on a codebase that doesn't even build. Exuberant Ctags dominated for twenty years until development stalled; Universal Ctags is the 2014 fork that carries on. Still worth generating for large, mixed or old projects.