Is “reoccurring” a word and is there any semantic difference with "recurring"?

Solution 1:

Actually, Merriam-Webster's Eleventh Collegiate Dictionary (2003) does have entries for both reoccur and reoccurrence—but it lists them under the entry for the prefix re-, which is where it puts words whose meanings differ from the root word(s) (in this case, occur and occurrence) only in adding "again" to each definition.

The full-size Merriam-Webster's Third New International Dictionary (1961/1986), having far more room to accommodate words of this type, includes the following entries:

recur ... vi recurred; recurred; recurring; recurs ... 1 : to return to a place or status [example omitted] 2 : to have recourse : go for help : RESORT [example omitted] 3 : to go back in thought or discourse [example omitted] 4 : to come up again for consideration : confront one again [example omitted] 5 : to come again to mind : return vividly to the memory [example omitted] 6 : to happen, take place, or appear again : occur again usu. after a stated interval or according to some regular rule [examples omitted] 7 : to repeat itself usu. indefinitely in fixed periods of figures (as of a decimal)

...

reoccur vi {re- + occur} : to occur again

reoccurrence n {re- + occurrence} : a second or another occurrence

From this treatment it seems clear that Merriam-Webster not only considers reoccur to be a real word, but also considers reoccurred, reoccurring, and reoccurs to be real words.

As for the difference in meaning, clearly recur has some unusual meanings that reoccur does not; but reoccur has more meanings than you might suppose from a casual scan of the three-word definition that the Third New International gives for it. Specifically, if it means "to occur again," it can apply to each of the first three general definitions of occur that appear in the entry for that verb—namely,

occur ... vi ... 1 : to be present or met with : EXIST [example omitted] 2 : to present itself : come to pass : take place : HAPPEN [example omitted] 3 : to come to mind : suggest itself [example omitted]

Add "again" to the end of each of those three definitions, and you have a fairly clear picture of what reoccur means according to Merriam-Webster.

I see considerable overlap between MW's definitions 4, 5, and 6 of recur and its implied definitions 1, 2, and 3 of reoccur, although the element of periodicity is certainly far stronger with recur.

Solution 2:

The Collins Dictionnary agrees with The Grammarist:

reoccur [ˌriːəˈkɜː] vb -curs, -curring, -curred (intr) to happen, take place, or come about again reoccurrence n Collins English Dictionary – Complete and Unabridged © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003

recur [rɪˈkɜː] vb -curs, -curring, -curred (intr) 1. to happen again, esp at regular intervals 2. (of a thought, idea, etc.) to come back to the mind 3. (of a problem, etc.) to come up again 4. (Mathematics) Maths (of a digit or group of digits) to be repeated an infinite number of times at the end of a decimal fraction [from Latin recurrere, from re- + currere to run] recurring adj recurringly adv Collins English Dictionary – Complete and Unabridged © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003

Emphasis mine.

And I have heard both used with this distinction.