After doing more research, what I really need is:
|stats count by Events |lookup EventList EC AS Events OUTPUTNEW Result AS MSG| (I only want MSG=Definition Not Defined)
Generally you could run a subsearch with your CSV lookup table to filter your indexed data (e.g. index=foo sourcetype=bar NOT [| inputlookup yourLookup.csv | table field1 field1.. | format]
). With little information in the question about your lookup table, your indexed data search and their relationship, I would suggest give something like above a try. Providing more information in your question would yield you better suggestion from the community.