The English language is in a constant state of change. The grammar you learned in high school or college becomes increasing obsolete each day. The main reasons for the accelerated rate of change? The Internet and text messaging.
Nothing demonstrates the change more than the fact that the publishers of the Shorter Oxford English Dictionary has removed hyphens from 16,000 words in its latest edition. For the most part, the dictionary dropped hyphens from compound nouns, which were unified in a single word (e.g. pigeonhole) or split into two (e.g. test tube), according to a report by Reuters.
Is there a rule you can apply to know if words have been unified or split? Nope. You’ll need to look up each word in the two-volume Shorter Oxford English Dictionary to know for sure.