A charge for dealing that is set out as its own line, separate from the spread, and usually quoted per lot.
It may be charged once, or on each side of a trade: on opening and again on closing. An account described as commission-free carries its dealing cost inside a wider spread instead.
commission = rate per lot × lots × sides charged
Worked example
A commission of 3.50 US dollars per lot, per side, comes to 7.00 US dollars for opening and closing one lot.
Educational information, not investment advice or a recommendation to trade.