In investing, my understanding is the opposite of a long is a short.

Shorting has asymmetric risk.

When you are long your loss is 100%. With a short it is theoretically unlimited.

> Shorting has asymmetric risk.

Shorting is perfectly symmetric with longing (except linguistically -- because "shorting" is perfectly normal, but "longing" not): in the former you have unlimited downside risk and limited upside set by the price at which you sold, in the latter you have unlimited upside potential and limited downside set by the price at which you bought.

Isn't this the situation that a margin call is meant to handle -- limiting your losses to the collateral you have on deposit?

The opposite is to borrow and then sell it.

That has a lot more risk.

https://en.wikipedia.org/wiki/Short_squeeze

Short selling a stock.