An edit summary is a summary of an edit you make. You can write it in the Summary box under an edit window.
The text written here, up to 200 characters, will appear in italics on the Recent changes page and in the page revision history (see also below). You should write a short summary of the changes you have introduced into the article. In addition (or alternatively), the summary field could explain why the change to the article was made, in order to allay the concerns of other contributors (in the case of potentially controversial or confusing edits). Longer explanations should go on the talk pages.
Always try to fill the summary. In the day-to-day operations, with a dozen (or hopefully more!) people all making changes at various times, it's better to have some summary of the changes rather than no summary at all. The summary can help people decide whether they should go read all of the changes that were made in your edit, or whether it's not important enough. (This would be in addition to the "minor edit" feature. See also Help: Editing.)
Efficient contributors to Wikipedia have developed a number of abbreviations for the summary field: 
Avoid "-" (single hyphen) as separation sign, it gives confusion with the meaning "remove".
"Post a comment" feature
When starting a new thread on a Talk page, optionally the "Post a comment" feature can be used. Then the edit summary is automatically the same as the new section header.
Places where the edit summary appears
The following pages generated by the MediaWiki software will display the edit summaries:
The Wikisimpsons search can not search edit summaries, and they are not indexed by Google.
When uploading an image, you can add an upload summary. If the upload is the first for a specific file name (i.e. it's creating a new Image: page), then the upload summary will also serve as the image description until it is edited. In all cases, the upload summary will be applied to the individual images that are uploaded.
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