Comments of the form % !TEX ... that can be used as directives in many tex editors
Magic comments are comments of the form % !TEX ... that can be used as directives in many tex editors, for example to specify (from within the document body) the root document, which tex engine should be used, the encoding, the language for spell checking etc.
An overview which editors support which magic comments can be found in the answer https://tex.stackexchange.com/a/84687/36296
