create array, use $.each to loop, and add value if not in array

You better use :

if(arr.indexOf({key: spanishWord, value: englishWord}) != -1){
    //do nothing
}
else{
    //push it
}

You can probably just use a javascript object here:

var dict = {};
y = 0;
$("a.defined").each(function() {
    var spanishWord = this.text;
    var englishWord = this.title;
    dict[spanishWord] = englishWord;
}

And there isn't really a need for unique checks, since newer values will just overwrite the older ones. If you don't want that behaviour, you can do this:

var dict = {};
y = 0;
$("a.defined").each(function() {
    var spanishWord = this.text;
    var englishWord = this.title;
    if (!(spanishWOrd in dict)) {
        dict[spanishWord] = englishWord;
    }
}

Javascript's in operator is not used for testing inclusion, it's used for iteration in a for .. in .. loop.

Other answers have suggested correctly that you need either .indexOf or JQuery's $.inArray method to test inclusion in an array, but there is a simpler (and faster) way of solving this problem: use a dictionary of key/value pairs instead!

var dict = {};
$("a.defined").each(function() {
    dict[this.textContent] = this.title;
});

Afterwards, you can use for key in dict to iterate over the list of unique Spanish words, and dict[key] to get the corresponding English translation.

Try this:

JavaScript

var arr = {};
$("a").each(function() {
var spanishWord = this.text;
var englishWord = this.title;
if(spanishWord in arr) {
    console.log("do nothing");
} else {
    arr[spanishWord] = englishWord;
}
});
console.log(arr);