The new beverage was regarded as a (18 - NOVEL) ………. at first, but coffee houses soon grew to rival the ubiquitous taverns in popularity

The appropriate word to fill in the blank is "novelty."

Explanation:

1. Context: The sentence discusses a new beverage that initially was not widely recognized but eventually gained significant popularity, particularly in comparison to taverns.
2. Word Formation: The word "novel" suggests something new or unique, while the suffix "-ty" denotes a state or condition. Thus, "novelty" refers to the quality of being new or unusual.

Final Answer:

The sentence would read: "The new beverage was regarded as a novelty at first, but coffee houses soon grew to rival the ubiquitous taverns in popularity."