Which of these product names has officially entered linguistic dictionaries as a generic term for its product category, causing concern for the company's trademark protection?

In the world of language and linguistics, product names can become part of our everyday vocabulary and even influence language evolution. Some linguistic terms have even been derived from famous brand names through a process called 'genericization' or 'proprietary eponymy.' Test your knowledge about this fascinating intersection of linguistics and marketing with this trivia question!

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Kleenex (facial tissues)
Windex (glass cleaner)
Post-it (sticky notes)
Sharpie (permanent marker)