If you wish to register a keyword as a trademark, it must meet multiple requirements.
If the keyword is non-descriptive of your goods and services and, in that sense, unique, it can likely be trademarked.
Your chances also depend on how generic the keyword in question is. If, for example, you wish to trademark the word "appliance" for your business selling fridges, the application will most likely receive a rejection. You cannot claim exclusive ownership of words that any member of the general public can use.
However, Apple Inc. registered the word "Apple" even though it is generic. In this case, the word "apple" is not descriptive of what the company does. This allows clients of Apple to associate the word "Apple" in the context of electronics solely with their brand. As you can see, the context is very important when trademarking a keyword or a generic word.