Less common vocabulary

The IELTS public band descriptors for speaking state that in order to achieve band 7 or above for Lexical Resource (LR - vocabulary), then you should use some less common vocabulary.

It's so hot today. I'm parched!

In this sentence, parched is an example of less common, or low-frequency vocabulary. If you use a word like parched, the IELTS examiner will begin to consider you as potentially band 7 or above.

Band 7 candidates, even band 8 candidates, might not know parched. Perhaps it is new to you today?

An example of a typically band 7 low-frequency word might be plummet:

In recent months, sales have plummeted.

Word forms and frequency

Sometimes one form of a word is frequent, while another form is less frequent.

I'm still very young, and so domesticity is still a long way away for me!
(less frequent than domestic)
My government definitely needs to think harder about inclusivity.
(less frequent than inclusive, which is less frequent than include)
Presidential candidates in my country like to demonstrate their religiosity.
(less frequent than religious)

Notice that in each of these examples, the low-frequency word form is the noun form! In a lot of academic writing, the noun form is more useful than other word forms, because it can be strategically located at the beginning of a sentence in what is known as theme position.

Vocabulary from your field

If you are scholar preparing for study abroad, then you have probably acquired a large vocabulary of low-frequency items related to your academic specialisation. If the examiner asks questions related to your field, use this vocabulary!

It would be fair to describe the following as band 9 items:

I don't really like music that is aleatoric.
(when asked about music you do not like, a specialist term from the field of music)
I prefer calisthenic training rather than using weights for strength.
(when asked about what kind of exercise you do, a specialist term from the field of health)
In my work I specialise in cantilevers.
(when asked about the kind of work you do, a specialist term from the field of engineering / architecture)

The importance of reading

An awareness of the frequency of vocabulary items comes through reading. When you read a novel, and you see a word often, then that word is a high-frequency word.