What is Pension Fund?

Legal Definition
A pension fund, also known as a superannuation fund in some countries, is any plan, fund, or scheme which provides retirement income.

Pension funds typically have large amounts of money to invest and are the major investors in listed and private companies. They are especially important to the stock market where large institutional investors dominate. The largest 300 pension funds collectively hold about $6 trillion in assets. In January 2008, The Economist reported that Morgan Stanley estimates that pension funds worldwide hold over US$20 trillion in assets, the largest for any category of investor ahead of mutual funds, insurance companies, currency reserves, sovereign wealth funds, hedge funds, or private equity.

The Federal Old-age and Survivors Insurance Trust Fund is the world's largest public pension fund which oversees $2.645 trillion USD in assets.
-- Wikipedia
Legal Definition
A contribution made by pooling up the resources, usually set up by unions, employers and various other organizations over a given period of time. It is used to provide the retirement benefits for the employees.