A pen lid is a small yet fundamental part of a pen that covers the writing nib to protect it from damage. There are several synonyms for a pen lid, including cap, stopper, cover, top, and plug. Caps may come with different shapes, such as snap-on, twist-on, or push-on caps. Plugs are typically a tight fit for a pen's nib to prevent ink from drying out. The term top is commonly used in fountain pens to refer to the part of the pen that screws in over the nib. Whatever the preference, having a secure lid is essential to ensure a pen's longevity and quality of writing.