I searched and posted a short list of various definitions I found on the web on "informal learning" because this term just seems to be wrong in some way.
I always believe that learning is an effort, an explicitly executed effort by the learner. To me, one of the most important measure of the effect of learning is whether [the result of a previous learning] can be called upon [in the future] to service a need.
This does not mean that the "effort" must be great and hard. It can actually be very enjoyable and satisfying. Just like the effort I am putting in now to try to understand what is "informal learning" and hence will improve the way I teach (not likely any more) or design my learnware.
Of course, such a definition of learning will post a problem for unintentional learning. Can unintentional learning really happen?
To me, informal learning refers to learning that is not officially organised. "Official" here refers to your employer, or if you are a student your school.
A note on just-in-time learning: Again I think there is something wrong with this term. When we face a problem, we call upon our "search skill" to find the information needed to solve the problem. The information is NOT just-in-time learning. The information is the result of an action which we have learnt before - the search skill. How many times we search for the same piece of information to solve the same problem? It is because we DID NOT MAKE an effort to take in that piece of information (ie we did not learn) so that we need to search for the same piece of information over and over again. The learnt skill that was used in this situation is the "search skill" which has been learnt before!