Spread

The gap between the best price buyers offer (the bid) and the best price sellers ask (the ask).

If the bid is $49.98 and the ask is $50.02, the spread is $0.04. Buying at the ask and selling straight away at the bid would lose you that gap, before any fee.

Shares that trade a lot usually have narrow spreads; thinly traded shares, and all shares outside regular hours, usually have wider ones.