Underweight

See: Overweight.

Professionally managed mutual funds have index grids against which they're measured. A common one is the S&P 500. So...let's say AAPL comprises 2% of the S&P500. A PM has to decide whether she wants to be overweight, underweight, or even...weight. Like...if her total AUM or Assets Under Management is $100 million, then a market weight weighting of AAPL would be a position of about $2 million. Underweight would be, well, less than $2 million.

In essence, being underweight a position is the mutual fund equivalent of being "short" that position.

Underweight. Under long.