comment
Java
A piece of text intended for the human reader of a program. Compilers ignore their contents.
In an XML document, text that is ignored unless the parser is specifically told to recognize it.
In a program, explanatory text that is ignored by the compiler. In programs written in the Java programming language, comments are delimited using // or /*...*/.
Perl
A remark that doesn't affect the meaning of the program. In Perl, a comment is introduced by a # character and continues to the end of the line.
SEO / Search Marketing
The HTML <!-- and --> tags are used to hide text from browsers. Some search engines ignore text between these symbols but others index such text as if the comment tags were not there. Comments are often used to hide javascript code from non-compliant browsers, and sometimes to provide invisible keywords to some search engines.