Heading: a heading is a component in a document that describes the subject of the section it is introducing. While the word likely brings up thoughts of newspaper headlines, essay headings, and other related items, in this case, the word describes an HTML element used to express and organize information expressed online. For example, the title of this glossary definition would be considered a heading.
In the HTML markup language, heading elements go from H1 to H6, with lower-numbers corresponding to more important headings. Headings are considered block elements in the HTML language, meaning that they will be presented as rectangular structure which spans the entire width of the parent element. This rectangular structure is also referred to as the box model, and describes several other components of an online document, including the content, border and margin.
Because headings describe the subject of the section they introduce, it is important that headings be relevant to the content, but also compelling. Likewise, all words should be spelled correctly, while sentences should use proper grammar and punctuation. However, length can vary from a single word to an entire sentence depending on the tone and style of the content.