Index: wp-includes/js/tinymce/langs/wp-langs.php
===================================================================
--- wp-includes/js/tinymce/langs/wp-langs.php	(revision 21260)
+++ wp-includes/js/tinymce/langs/wp-langs.php	(working copy)
@@ -542,7 +542,7 @@
 		's130' => __('130%'),
 		'img_title' => __('Title'),
 		'caption' => __('Caption'),
-		'alt' => __('Alternate Text')
+		'alt' => __('Alternative Text')
 	);
 
 	$locale = _WP_Editors::$mce_locale;
Index: wp-admin/includes/media.php
===================================================================
--- wp-admin/includes/media.php	(revision 21260)
+++ wp-admin/includes/media.php	(working copy)
@@ -812,7 +812,7 @@
 
 		$form_fields['image_alt'] = array(
 			'value' => $alt,
-			'label' => __('Alternate Text'),
+			'label' => __('Alternative Text'),
 			'helps' => __('Alt text for the image, e.g. “The Mona Lisa”')
 		);
 
@@ -1979,7 +1979,7 @@
 
 		<tr class="image-only">
 			<th valign="top" scope="row" class="label">
-				<span class="alignleft"><label for="alt">' . __('Alternate Text') . '</label></span>
+				<span class="alignleft"><label for="alt">' . __('Alternative Text') . '</label></span>
 			</th>
 			<td class="field"><input id="alt" name="alt" value="" type="text" aria-required="true" />
 			<p class="help">' . __('Alt text for the image, e.g. &#8220;The Mona Lisa&#8221;') . '</p></td>