TITLE: What's meaning of the inverted Greek letter iota “ι” in Principia Mathematica I* 14?
QUESTION [6 upvotes]: The inverted iota always followed by a variable as following:
(ιx)ϕx
Some book say the formula should be read "The x such that x is ϕ". Does it is meaning as following:
x=ϕ
assign ϕ to variable x?

REPLY [10 votes]: This is the descriptor operator. $(\iota x)\varphi x $ is the unique $x $ with the property specified by $\varphi $ (should it be the case that, indeed, there is precisely one such $x $). The Wikipedia entry on Principia has a very decent explanation of their notation.