time frame or timeframe?

Should time frame be written as one word or two? At present, it is most commonly styled as two words with no hyphen in between.

Compounds, or “words consisting of components that are words,” come in three varieties: open (two words with a space between them, such as lima bean), hyphenated (two words with a hyphen between them, such as runner-up), and closed (one word, such as rowboat). In some cases, competing compound stylings appear in different publications. This is generally acceptable as long as the spelling remains consistent throughout a book or periodical.

Compounds have a tendency to move from open to closed as time goes by, and our evidence suggests that time frame is headed in this direction. At this point, however, the open compound is far more prevalent.