Can a system file be deleted?

Hi #-Link-Snipped-#, welcome to CE.

I'm assuming you asked this query from the perspective of Operating Systems. Basically, system files are those which are responsible for working of your operating system. Just like you write programs for performing various tasks and operations, system files altogether makes your system work.

Usually, system files are hidden as they are under execution when the system is running and obviously system does not want these files to be modified by user. Though in some cases, user can delete/modify system files which can lead to partial/complete failure of system or the system may behave abnormally.
In Windows environment, system files have extension as .sys. System considers files with .sys extension as important, but it doesn't necessarily make them system files. Hope this clarify your questions.