Magic
I'm sure there are all sorts of almanacs with custom notation that talk about magic, but this page exists to talk about (and link to/from elsewhere on the wiki where needed) the convention of (typically) using the first few bytes of a file to notate/detect the reported format, "as is by magic" (actually by table lookups).
Magic is a string of unique hexadecimal pattern(s) to look for, used in file as, "relatively quick-running heuristics to determine file type, it can report misleading information. The command can be fooled, for example, by including a magic number in the content even if the rest of the content does not match what the magic number indicates. The command report cannot be taken as completely trustworthy."[1] This method, however, is beneficial in somewhat quickly ascertaining whether a or an (extension-less) file is human-readable, or machine code, as opposed to using utilities such as cat to show the contents of a binary file to terminal, can ruin it due to the use of control characters.[2]
It is also used as a library containing a database of magic numbers that can be used, for instance binwalk to, "identify, and optionally extract, files and data that have been embedded inside of other files"[3], based on libmagic signatures.
