You may have noticed that several popular startups have names that are also everyday words, such as "Coin" or "Slack," rather than made up or modified words like "Facebook" or "Tumblr." How is it that they can use these everyday words as their brand names?
First, as a really short primer on trademarks: their purpose is to connect certain words, logos or short phrases to a brand, allowing the general public to trust in a particular level of quality by consuming goods or services branded by that certain company's mark.
Everyday words are usually not yet associated with a brand. The fact that a word is already in common vernacular does not alone make it "owned" or "trademarked" by anyone. In fact, common words, used in their dictionary-defined, traditional context are generally not eligible for intellectual property protection (for example, the word "basket" to sell baskets). The companies mentioned above have taken common words and applied them in the context of selling something other than the exact words themselves - Coin is not selling coins, but rather, using technology to create a consolidated way for people to use their credit cards. That creative application is what provides the basis for intellectual property rights.
Lastly, this is a big reason why people (should) hire lawyers when creating their branding. Trademark lawyers will perform extensive searches to determine whether or not the name of a company is available for use and trademark registration. Note, however, that more than one company can use the same word as a trademark as long as the one company's products would not be mistaken as coming from the other. Look at Delta Airlines and Delta faucets - it is highly unlikely that the general public would assume that the airline is also manufacturing faucets, and vice versa.
This is how brands and everyday words can live in harmony.
[Originally posted in modified form on Quora.com]