A "×" symbol (a cross similar to the letter "x") in a box.
Note that the filename is "☒", which is the Unicode character for the symbol in question (U+2612).  If your computer displays something like a question mark ("?") instead, then you have a browser problem.  But don't worry.  You can still copy and paste [[File:☒.svg]] into an article and it will appear correctly.

Don't be deceived by the tiny size of this image.  It is fully scalable.
Own work
Erin Silversmith