Simple Definition:

A website is a collection of web pages [pages on the internet] that are usually connected to each other and can be accessed through the internet using a web browser [a program that lets you browse the internet].

For example, Google.com, Facebook.com, and Amazon.com are all websites.

Very Simple Definition:

A website is a place on the internet that you can visit using a special program called a web browser.

For example, Google is a website where you can search for things.