The Placeholder Idiom uses an underscore (_) to indicate an unused parameter. jQuery uses this idiom also. Here are two code snippets from its source: the first one has a placeholder parameter in a callback function. The second uses it in a try/catch block:

// Listener
callback = function( _, isAbort ) {
    // etc.
}

And

// Need an extra try/catch for cross domain requests in Firefox 3
try {
    for ( i in headers ) {
        xhr.setRequestHeader( i, headers[ i ] );
    }
} catch( _ ) {}

Placeholder is a simple but helpful idiom.