In some functional languages, partially applied functions and curried functions are two similar but distinct concepts.
Reading the book Learn You a Haskell for Great Good, the author appears to apply these interchangeably.
[Curried functions] Every function in Haskell officially only takes one parameter.
...
if we call a function with too few parameters, we get back a partially applied function, meaning a function that takes as many parameters as we left out.
My question is: Are the concepts of partial application and currying interchangeable in Haskell?
They are different but related concepts. Because function definitions are curried, when you partially apply a function (ie, call it with fewer arguments than it expects), you get back another function that expects fewer arguments.
The process of currying is replacing an N-ary function with N "layers" of unary functions so that you can partially apply it, turning
function(x,y) {return x+y;}
into
function(x) {return function(y) {return x+y;};}
The currying is part of defining the function, and partial application is what you do with a function when you call it.