This guideline is a part of the English Wikipedia's Manual of Style.
|This page in a nutshell: The lead should identify the topic and summarize the body of the article with appropriate weight.|
|Manual of Style (MoS)|
The lead section (also known as the lead or introduction) of a Wikipedia article is the section before the table of contents and the first heading. The lead serves as an introduction to the article and a summary of its most important contents. It is not a news-style lead or "lede" paragraph.
The average Wikipedia visit is a few minutes long. The lead is the first thing most people will read upon arriving at an article, and may be the only portion of the article that they read. It gives the basics in a nutshell and cultivates interest in reading on—though not by teasing the reader or hinting at what follows. It should be written in a clear, accessible style with a neutral point of view.
The lead should stand on its own as a concise overview of the article's topic. It should identify the topic, establish context, explain why the topic is notable, and summarize the most important points, including any prominent controversies. The notability of the article's subject is usually established in the first few sentences. As in the body of the article itself, the emphasis given to material in the lead should roughly reflect its importance to the topic, according to reliable, published sources. Apart from basic facts, significant information should not appear in the lead if it is not covered in the remainder of the article.
As a general rule of thumb, a lead section should contain no more than four well-composed paragraphs and be carefully sourced as appropriate.