<p>The Courant Institute is a department within the College of Arts and Science, so if you would like to apply as a math major, then you would apply to CAS. While you’re filling out your Common Application and NYU Supplement, you would mark that you’re applying to CAS as a Math major. As far as I know, there is no direct application to the department, as that isn’t the case for any major.</p>
<p>Edit: The link you provided gives a list of all the colleges within NYU. Try thinking of the Courant Institute as simply the “math” part of CAS, just as the Economics department would be the economics part.</p>