The term expressing opposition is 'adversative, ' which is an 11-letter word. Adversative is an adjective that describes a situation or action that expresses opposition or disagreement. It is often used to describe words, phrases, and sentences that are used to express opposition or contrast. For example, 'He was adamant in his adversative stance on the issue.' Adversative can also be used to describe a person's attitude or behavior that expresses opposition or disagreement.