The crossword solution for the term SUFFIX USED TO FORM ABSTRACT NOUNS is TION, which is a four-letter suffix. This suffix is commonly used to form abstract nouns from verbs, adjectives, and other nouns. For example, the verb 'CREATE' can be turned into the abstract noun 'CREATION' by adding the suffix -tion. Similarly, the adjective 'HAPPY' can be turned into the abstract noun 'HAPPINESS' with the same suffix. This suffix is also used to form nouns from other nouns, such as the noun 'NATION' becoming the abstract noun 'NATIONHOOD' with the addition of -tion.