Proxy
From ACT Wiki
1.
A person authorised to act for another.
2.
In relation to a company, a person empowered by a shareholder to vote on the shareholder's behalf at company meetings.
3.
A readily available investment whose characteristics are strongly positively correlated with those of another - not so readily available - investment.
4.
A readily observable measure which is strongly positively correlated with another measure of interest, but less readily observable.