Complex describes something with many connected parts that work together in ways that aren't immediately obvious. A computer is complex: it contains millions of tiny components that must coordinate perfectly for it to function. The human body is complex: your brain, heart, lungs, and thousands of other parts work together in intricate ways that scientists are still figuring out.

When something is complex, you can't understand it just by looking at the surface. A simple machine like a lever has one moving part that's easy to grasp. A complex machine like a car engine has hundreds of parts that interact in multiple ways. Understanding how they all fit together takes real study and thought.

In everyday conversation, people sometimes confuse complex with complicated, but they're slightly different. Something complicated is difficult and confusing. Something complex has many interconnected parts but might actually work elegantly once you understand it. A symphony orchestra is complex (many instruments playing different notes that blend into beautiful music) but not necessarily complicated to enjoy.

The word can also refer to a group of related buildings or facilities, like an apartment complex or a sports complex. And in psychology, a complex is a related group of thoughts or feelings about something, like having a complex about being the youngest sibling or about your height.