A fixed per-lot or per-trade fee charged by the broker in addition to the spread, common on ECN and raw-spread accounts.
ECN brokers offer near-zero raw spreads but charge a round-turn commission (e.g. USD 7 per standard lot). Adding the commission-equivalent spread back makes the true cost comparable with a wider-spread, commission-free account.
When comparing broker costs, traders should calculate total cost per lot: effective spread + commission. Commission-based accounts often work out cheaper for active traders than spread-only accounts.