# 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.
#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 Section 2.2.5.
New in Version 1.