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In the context of **Node.js**, the term **I/O** stands for **Input/Output**, meaning **data going in and out**.
It's a general label for any operation involving a program interacting with the outside world:

- reading and writing files (via `fs`),
- database queries,
- network requests (HTTP, TCP, WebSocket, and so on),
- console or user interactions.

### The core idea:

Node.js is built around **non-blocking (asynchronous) I/O**.
That means when a program does, say, a file read or an HTTP request, it **doesn't wait** for the operation to finish, instead:

1. It starts the operation (for example, reading a file);
2. It hands control back;
3. Once the operation finishes, a **callback**, **Promise**, or **async/await** handles the result.

### Example:

```javascript
const fs = require('fs');

// Asynchronous (non-blocking) file read:
fs.readFile('data.txt', 'utf8', (err, data) => {
  if (err) throw err;
  console.log(data);
});

console.log('This log prints before the file's content does');
```

Here, Node.js doesn't wait for the file read to finish, it keeps running the rest of the code, an example of **asynchronous I/O**.

### Why this matters:

- It doesn't block the execution thread.
- It lets Node.js serve thousands of concurrent requests.
- It's the foundation of high performance for network and file work.

**Summary:**

> **I/O (Input/Output)** in Node.js covers input/output operations (files, network, databases, and so on), implemented asynchronously and non-blockingly, to make the most of a single-threaded architecture.

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