Here is an interesting definition for Keyword Cannibalization in Google.com :-)

Have simplified the cannibalized definition portrayed above:
Keyword Cannibalization in SEO:
Targeting the same keywords for different pages thereby reducing the visibility for all; except one, which the search engines might consider for that query.
Keyword Cannibalization in Campaign Management:
Having the same keywords in different Adgroups. Only one keyword gets a chance to show-up from one account, thereby creating a tug-of-war among the keywords.

Have simplified the cannibalized definition portrayed above:
Keyword Cannibalization in SEO:
Targeting the same keywords for different pages thereby reducing the visibility for all; except one, which the search engines might consider for that query.
Keyword Cannibalization in Campaign Management:
Having the same keywords in different Adgroups. Only one keyword gets a chance to show-up from one account, thereby creating a tug-of-war among the keywords.