SOLID is a set of five principles for writing object-oriented code that holds up well over time. The name is an acronym, with each letter standing for one rule about how to structure your code so that future changes are easy rather than risky. They were popularised by developer Robert Martin and have become a common reference point on software teams.
The clearest way to picture SOLID is a well-organised toolbox. Each tool has one clear job, the drawers are labelled, and you can swap a worn screwdriver for a new one without rebuilding the whole box. Code that follows SOLID behaves the same way: each part does one thing, pieces can be replaced without breaking their neighbours, and adding a feature does not mean tearing the rest apart. These ideas sit at the heart of clean code and lean heavily on solid object-oriented programming.
To make one letter concrete: the S, Single Responsibility, says a class should have one reason to change. A class that both formats an invoice and emails it has two jobs, so a change to the email rules risks breaking the formatting. Split them and each can move on its own. The other four letters chase the same goal from different angles.
The principles are guidelines, not commandments. Applied with judgement they keep a large codebase calm and changeable. Applied rigidly to a tiny script, they just add ceremony. Knowing when to use them is the real skill.
At TopDevs we follow the SOLID principles where they keep your software cheap to change, so adding a feature next year does not mean rebuilding what already works.
