Slot

# represents the first argument supplied to a pure function.

#n represents the n argument.

# is used to represent arguments or formal parameters in pure functions of the form body& or Function[body].

# is equivalent to Slot[1].

#n is equivalent to Slot[n]. n must be a non-negative integer.

#0 gives the head of the function, i.e., the pure function itself.

See The Mathematica Book: Section 2.2.5.

Further Examples