The term SUFFIX FOR 'MOUNTAIN' OR 'AUCTION' can be solved by two different words, EER and EERS. EER is a three-letter suffix that is commonly used to form nouns from adjectives or verbs, and it is often used to refer to a person who performs an action. EERS is a four-letter suffix that is also used to form nouns from adjectives or verbs, but it is usually used to refer to a group of people who perform an action. Both EER and EERS are commonly used as suffixes for the words 'MOUNTAIN' and 'AUCTION', and they are used to refer to a person or group of people who perform an action related to the word.