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A hash making the rest of a line invisible to the interpreter.
In shell scripts and in many scripting languages a hash marks everything after it on the line as a comment, ignored when the program runs.
POSIX defines a word beginning with the hash character as introducing a comment, with the rest of the line ignored.
Contexts: Software
The same convention is followed by Python, Ruby, Perl and most configuration file formats, which is why it reads as 'comment' almost anywhere.
Contexts: Software
Two horizontal strokes crossed by two slightly sloping upright strokes, forming a small grid with four open corners.
The same character. The number sign is the typographic mark; this is one of the jobs programming languages give it.
A shebang line begins with this same hash, which is why the shell reads it as a comment while the kernel reads it as an instruction.
Both are built from lines and dots, and both are used in software contexts.