HTML is called a markup language because it’s mainly used to structure and present content on a webpage, not to perform logic or calculations like a programming language does.

For example, HTML lets you define elements like headings, paragraphs, images, and links, but it can’t make decisions, loop, or perform calculations—things you would typically expect from a programming language like JavaScript, Python, or C++.

In HTML, you might write:

Welcome to my website!
This is a simple paragraph.
Visit Example

This tells the browser what to display and how to organize it, but there’s no logic like "if this happens, then do that".

In contrast, in a programming language like JavaScript, you could write:

let age = 18;
if (age >= 18) {
    console.log("You are an adult.");
}

Here, the code makes a decision based on the value of age, which HTML alone can't do.