The term third-level domain name refers to a domain that is in the DNS hierarchy directly below a second-level domain name. 

• .uk - this is a top-level domain name, also known as TLD. 
• example.uk - this is a second-level domain name. 
• example.co.uk - this is a third-level domain name. 

In the UK there are 9.4 million third-level domains registered compared to just 2.4 million second-level domains. [Correct as of February 2020]