Commission
A fixed fee per trade charged by the broker or exchange; on ECN accounts, a tight spread plus a per-lot commission is a common model.
On forex ECN accounts the typical fee is $5-7 per lot round-turn; in futures, it's a per-contract fee (exchange + clearing + broker). Crypto exchanges apply a maker/taker percentage.
Total cost = spread + commission + swap + slippage. Looking only at "commission-free" is misleading; commission-free accounts bury the cost in the spread.