But what does “less common” mean? If you are a language teacher, I can tell you that it is C1 level lexis – the type that is taught in any good Advanced text book (such as English File or Speak Out). But, if you are a student, it is very hard for me to summarise what is considered “less common”, as you don’t know the lexis that we teach to students at different levels (i.e. Beginner, Intermediate, Upper Intermediate, etc).
What I can tell you is that it does not mean that these words are less common for native speakers. This is a common misconception and why many students think that it is a good idea to use a thesaurus to find the most difficult version of a word that they can (as I explain later, this is never a good idea). No, it means that they are less common words for second-language speakers to produce. For example, if you are discussing the environment, you would expect most good speakers to produce words like “climate change” or “fossil fuels” but not so many to be able to produce words like “contaminated” or “sustainable development“.