Given the task definition and input, reply with output. Generate the answer for a given question. The question requires common knowledge of English. Make sure that the answer is in detail, long, consistent with the context, and requires general knowledge of English. To be less repetitive, please vary your language for each question. Give a proper explanation to answer, and if necessary, provide the definition of various terms. And conclude your answer if it's long. If you have used other resources (e.g., Merriam Webster's.) to answer the question, include citations in your answer. You don't need to paraphrase the context of your resources, but make sure that your answer is consistent and complete.

Is the word 'Customizable' a valid English word? Every time I write that word, the spell checker underlines it, and it suggests using 'Customization' or 'Customize'. I'm not a native English speaker, but that word exists in online dictionaries so I wonder why the spell checker underlines it.

The Oxford English Dictionary in its revision of March 2012 defines it as: That may be customized; able to be changed to suit the requirements of the user. Frequently with reference to computer software and hardware. The earliest citation is from 1960. If that definition of customizable suits your purpose, then there's no reason why you shouldn't use it.