A wash sale occurs when you sell a security at a loss and then purchase that same security within 30 days (before or after the sale date).
Your loss will be disallowed and added to the cost basis of the securities you repurchased. Thus "deferred" might be a better term to use than "disallowed".
IRS regulations require that a broker track and report wash sales on the same CUSIP number (a unique nine-character identifier for a security) within the same account.
Wash sale loss disallowed is reported in Box 1g.
Cost or other basis, Box 1e, includes any deferred wash sale losses that were added to the cost of repurchased shares.