Origin: Latin suffix -ary
Subsidiary has 5 different meanings across 2 categories:
an assistant subject to the authority or control of another
"The younger brother acted as a subsidiary figure, always following his elder sibling's lead during their long hiking trip."
a company that is completely controlled by another company
"The new subsidiary operates under the full direction of its parent company."
A company owned by a parent company or a holding company, also called daughter company or sister company.
"The subsidiary was acquired last year and now operates under the direction of its parent corporation."
In plain English: A subsidiary is a smaller company that belongs to and is controlled by another larger business.
"The bank opened a new subsidiary in Tokyo to expand its business operations."
functioning in a supporting capacity
"the main library and its auxiliary branches"
Auxiliary or supplemental.
"The company launched a subsidiary program to offer additional benefits to its employees."
In plain English: Something that is subsidiary is not as important as other things and exists mainly to support them.
"The company bought several small subsidiary businesses to expand its market reach."
Usage: Use the adjective form to describe something that is secondary or auxiliary in importance rather than primary. This term often appears when referring to subsidiary branches of an organization or supporting details within a larger argument.
The word subsidiary comes from Middle French and originally meant belonging to a reserve. It entered English with this same sense of being secondary or supplementary in nature.