This sounds like one of those 'email or e-mail' questions people used to ask. Google gives 12,100,000 results for elearning and 46,400,000 results for e-learning, so I think at the moment e-learning is the way to go.

I don't know if there is a definitive answer and either is OK from the communication point of view.  For what it is worth Word 2003 and 2007 UK English spelling accepts e-learning and tells us elearning is a spelling mistake. So along with Google it is looking like e-learning is more popular especially if you don't want squiggly lines under your text when using Word.

I for one prefer online learning or just learning; interactive or web based learning are OK too. However, it may not be online and it may not be interactive, but it could still be computer based. Unfortunately CBT or Computer Based Training is terrible and is evocative of those CDs from the 1990s containing lots of worthy content which no one ever watched. So, best to avoid CBT; the future is e-learning.  Write in if you disagree.

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Mary who sent in a comment reckons:

Since it stands for electronic learning, I believe the hyphen is required until the slang version (without the hyphen) is accepted in the dictionary.

If you follow the technical writing protocols, then it really should be E-learning (with a capital 'E'); however, words morph so quickly now-a-days that lowercase words are accepted simply to save time (in texting).

Sometimes protocols are over rated.