Terminology: substring of a KW string

[unirmk]

Hi everyone,
I am struggling to remember the correct term to describe a substring of a KW phrase.

For example- the KW "top online seo tools" is composed of the substrings:
top online seo tools top online seo tools
top online seo tools top online seo tools top online seo tools

There is a term that describes those "KWs within KWs", and I am going crazy trying to remember what it is...

Thanks!