Index: src/wp-activate.php
===================================================================
--- src/wp-activate.php	(revision 25481)
+++ src/wp-activate.php	(working copy)
@@ -52,10 +52,13 @@
 add_action( 'wp_head', 'wpmu_activate_stylesheet' );
 
 get_header();
+
+$key_param = wp_activation_key_param();
+
 ?>
 
 <div id="content" class="widecolumn">
-	<?php if ( empty($_GET['key']) && empty($_POST['key']) ) { ?>
+	<?php if ( empty( $_GET[ $key_param ] ) && empty($_POST['key']) ) { ?>
 
 		<h2><?php _e('Activation Key Required') ?></h2>
 		<form name="activateform" id="activateform" method="post" action="<?php echo network_site_url('wp-activate.php'); ?>">
@@ -70,7 +73,7 @@
 
 	<?php } else {
 
-		$key = !empty($_GET['key']) ? $_GET['key'] : $_POST['key'];
+		$key = ! empty( $_GET[ $key_param ] ) ? $_GET[ $key_param ] : $_POST['key'];
 		$result = wpmu_activate_signup($key);
 		if ( is_wp_error($result) ) {
 			if ( 'already_active' == $result->get_error_code() || 'blog_taken' == $result->get_error_code() ) {
@@ -116,4 +119,4 @@
 	var key_input = document.getElementById('key');
 	key_input && key_input.focus();
 </script>
-<?php get_footer(); ?>
\ No newline at end of file
+<?php get_footer(); ?>
Index: src/wp-includes/ms-functions.php
===================================================================
--- src/wp-includes/ms-functions.php	(revision 25481)
+++ src/wp-includes/ms-functions.php	(working copy)
@@ -714,11 +714,13 @@
 	if ( !apply_filters('wpmu_signup_blog_notification', $domain, $path, $title, $user, $user_email, $key, $meta) )
 		return false;
 
+	$key_param = wp_activation_key_param();
+
 	// Send email with activation link.
 	if ( !is_subdomain_install() || $current_site->id != 1 )
-		$activate_url = network_site_url("wp-activate.php?key=$key");
+		$activate_url = network_site_url("wp-activate.php?$key_param=$key");
 	else
-		$activate_url = "http://{$domain}{$path}wp-activate.php?key=$key"; // @todo use *_url() API
+		$activate_url = "http://{$domain}{$path}wp-activate.php?$key_param=$key"; // @todo use *_url() API
 
 	$activate_url = esc_url($activate_url);
 	$admin_email = get_site_option( 'admin_email' );
@@ -773,6 +775,8 @@
 	if ( !apply_filters('wpmu_signup_user_notification', $user, $user_email, $key, $meta) )
 		return false;
 
+	$key_param = wp_activation_key_param();
+
 	// Send email with activation link.
 	$admin_email = get_site_option( 'admin_email' );
 	if ( $admin_email == '' )
@@ -784,7 +788,7 @@
 			__( "To activate your user, please click the following link:\n\n%s\n\nAfter you activate, you will receive *another email* with your login." ),
 			$user, $user_email, $key, $meta
 		),
-		site_url( "wp-activate.php?key=$key" )
+		site_url( "wp-activate.php?$key_param=$key" )
 	);
 	// TODO: Don't hard code activation link.
 	$subject = sprintf(
@@ -2059,3 +2063,20 @@
 
 	return $site_results;
 }
+
+/**
+ * Return the URL parameter to use for activation keys
+ *
+ * @since 3.7.0
+ *
+ * @return string The activation key paramater. Defaults to 'k'.
+ */
+function wp_activation_key_param() {
+	/**
+	 * Filter the URL parameter to use for activation keys
+	 *
+	 * @since 3.7.0
+	 * @param string The activation key parameter.
+	 */
+	return urlencode( apply_filters( 'wp_activation_key_param', 'key' ) );
+}