The word 'synonymousness' refers to the quality of being synonymous or interchangeable. There are many other words that express similar meanings to this term. A few of them include equivalent, identical, comparable, interchangeable, corresponding, and parallel. These words signify that two things are similar, and can be used interchangeably in certain circumstances. When we say that two words or phrases are synonymous, it means that they have the same meaning. Synonyms are useful to express ideas in different ways and can make language more interesting. So, whether you use 'synonymousness' or any of these synonyms, the end result will be a clear and concise expression of ideas.