It is called a pen cap
Do you know where the cap for this pen is? is an often heard question for small children after they've been drawing.
ell.stackexchange.com/users/26439/Peter
There are a few different names for it, which vary in preference depending on your locale.
I've heard pen cap and pen lid , and while doing some further research on this question top seems to be a word used as well.
For the most part they can be used interchangeably, though some people will insist lid is incorrect.
Lid is the natural word for me, so your mileage may vary.
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In England, pen lid tends to be preferred, with pen cap also being used.
This may be subject to regional or age difference though (Britain has a very high level of regional difference).
Pen top would be least common.
It would possibly be used to indicate the top end of a pen distinct from its bottom, but very unlikely to be used to refer to the lid itself.
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I would call this a cap in American English.
In American English
a lid is often thought of as a flat item that covers something.
It often snaps or holds in place somehow.
For instance, the floppy plastic cover on top of a can of peanuts would be a lid.
A cap has similar purpose to a lid, but it is slightly different in that a cap will typically completely encompass the object that it is covering, whereas a lid usually rests on top.
Webster's defines a cap this way: Something that serves as a cover or protection especially for a tip, knob, or end Source
ell.stackexchange.com/users/48982/Chris Schneider
Pen top is also used by native speakers.
As in: Where is the top of this pen ? and Have you seen my pen top ?
I've used it on two continents and no one has indicated my dialect usage is not natural.
ell.stackexchange.com/users/20600/Brian Tompsett - 汤莱恩
What is the cover of the head of a pen called in English?   In my native language it is simply called a cover.
What is the cover of the head of a pen called in English?
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ell.stackexchange.com/users/12430/Judicious Allure
Pen cap, pen top and pen lid are all used. In England,  pen lid  tends to be preferred, with  pen cap  also being used.
There are a few different names for it, which vary in preference depending on your locale. Lid is the natural word for me, so your mileage may vary. In England,  pen lid  tends to be preferred, with  pen cap  also being used. It would possibly be used to indicate the top end of a pen distinct from its bottom, but very unlikely to refer to the lid itself.