Core terms
ECN (electronic communication network)
An electronic trading system that displays orders entered into it and lets other participants trade against them. Traders also use "ECN fees" loosely for the per-share fees and rebates venues charge or pay on each fill.
ECNs began as separate electronic venues outside the traditional exchange floor; Regulation NMS defines the term in Rule 600. Over time many became or merged into exchanges, so most per-share fees today come from exchanges, but the trader's name "ECN fees" stuck.
For a scalper, the useful question is not the venue's legal status but what each fill costs: whether the order added or removed liquidity, the venue's fee or rebate for that, the broker's commission and the regulatory fees on sells. Added up, these set the cost per round trip and the minimum move a scalp needs to break even.
What traders should verify
Build a cost-per-round-trip worksheet from your plan's pass-through schedule, once for a take-and-add round trip and once for take-and-take, and note the date you checked it.
Do not conflate
"ECN fees" is trader usage for venue fees in general, not only fees charged by ECNs. Fee and rebate levels differ by venue and tier and change over time.
Primary source
Primary source17 CFR 242.600, Regulation NMS definitions