on behalf of someone Definition

behalf
noun

1. Now usually only used in the phrases

Idiom: in behalf of someone (in someone's behalf)

( Amer)
On behalf of someone

Idiom: on behalf of someone (on someone's behalf)

For, in or referring to their interests, or on their account.
Example: acting purely on his own behalf

Speaking or acting, etc as a representative of someone or something.

Etymology: 14c as behalve: from Anglo-Saxon be by + healfe side.

Info:

There is often confusion between on behalf of and on the part of: on behalf of means ‘acting as a representative of', wheareas on the part of simply means ‘by' or ‘of'.