"semi transparent", what is used in between?

How do you write the word semi transparent (meaning partially transparent)?

1. semitransparent
2. semi-transparent
3. semi transparent

I found each of them on the Internet and none of them in my English dictionary.

Solution 1:

The British National Corpus has 4 mentions of semitransparent, and none of either semi-transparent or semi transparent.

Wiktionary has an entry for semitransparent, but not for semi-transparent. Merriam-Webster has an entry for semitransparent and says that the first known use was in 1731.

(Google seems to be rather unreliable here: searching for "semi transparent" matches semitransparent and semi-transparent, and vice versa.)

Solution 2:

The American Heritage Dictionary Fourth Edition includes “semitransparent”.

“semi-”, in this case, is being used a prefix rather than a word. Because of this, I would not use “semi transparent”.

Regarding “semi-transparent”, my understanding is that the hyphen tends to be dropped and the word elements combined to form a derivative word according to how common the combination is. Therefore, if I had not been able to locate it in the dictionary, then I would likely have used the hyphenated prefix.