<ul><li><code>menu</code> shows the menu.</li><li><code>countdown</code> uses a text-mode countdown.</li><li><code>hidden</code> hides GRUB entirely.</li></ul><p>When using a theme, the default value (<code>menu</code>) is appropriate for the graphical countdown.</p><p>When attempting to do flicker-free boot, <code>hidden</code> should be used.</p><p>See the <a href="https://www.gnu.org/software/grub/manual/grub/html_node/timeout.html" rel="nofollow">GRUB documentation section about <code>timeout_style</code></a>.</p><div class="mnos-note">If this option is set to ‘countdown’ or ‘hidden’ [...] and ESC or F4 are pressed, or SHIFT is held down during that time, it will display the menu and wait for input.</div><p>From: <a href="https://www.gnu.org/software/grub/manual/grub/html_node/Simple-configuration.html" rel="nofollow">Simple configuration handling page, under GRUB_TIMEOUT_STYLE</a>.</p>

boot.loader.grub.timeoutStyle

Menu shows the menu.countdown uses a text-mode countdown.hidden hides GRUB entirely