JSLint: was used before it was defined
From the documentation
JSLint also recognizes a /*global */ directive that can indicate to JSLint that variables used in this file were defined in other files. The comment can contain a comma separated list of names. Each name can optionally be followed by a colon and either true or false, true indicating that the variable may be assigned to by this file, and false indicating that assignment is not allowed (which is the default). The directive respects function scope.
Some globals can be predefined for you. Select the Assume a browser (browser) option to predefine the standard global properties that are supplied by web browsers, such as document and addEventListener.
Example:
/*jslint browser: true*//*global $, jQuery*/
As Quentin says, there's a /*global*/ directive.
Here is an example (put this at the top of the file):
/*global var1,var2,var3,var4,var5*/Make sure the initial global statement is on the same line as /*, or else it breaks.