I'm not sure how correct my information is but I was once told that it should be written out in full but could be contracted when spoken. e.g. Written "is not" can be spoken "isn't"....... Or was it the other way round?!!
noun
1. a message that departs from the main subject
2. a turning aside (of your course or attention or concern); "a diversion from the main highway"; "a digression into irrelevant details"; "a deflection from his goal" [syn: diversion]
3. wandering from the main path of a journey