An exclusion is an instance of leaving something or someone out. If you love someone to the exclusion of all others, he or she is the only one for you!
Exclusion is closely related to some words that have a positive or negative feel. The word exclusive implies something is high-end, desirable, and not for ordinary people. If someone is excluded, on the other hand, it means that someone has been deliberately and painfully left out. Exclusion is more neutral. The department store that advertises a big sale often states at the bottom of the ad that "some exclusions apply," meaning the discounts don't apply to all items.