I like ClearAll better than Clr All.

IPython/frontend/html/notebook/templates/notebook.html

@@ -95,7 +95,7 b''
                     <span id="toggle_output" class="section_row_buttons">
                         <button id="collapse_cell">Collapse</button>
                         <button id="expand_cell">Expand</button>
-                        <button id="clear_all_output">Clr All</button>
+                        <button id="clear_all_output">ClearAll</button>
                     </span>
                     <span class="button_label">Output</span>
                 </div>