A customer important enough, by revenue, potential or strategic value, to get a named owner, a plan and its own review.
A key account is a customer the business has decided to manage individually rather than as part of a territory or segment. It gets a named owner, a written account plan, regular executive contact and its own revenue and margin reporting. Selection should rest on current revenue and on potential, so that a large customer where you hold a small share of wallet can qualify too.
Rank accounts by revenue, gross margin or estimated potential, set a cutoff (a top N, or a revenue floor), and report the group's share of total revenue each period alongside its growth.
Of 400 accounts with $14,000,000 of revenue, the top 25 bring in $6,300,000, which is 45% of the total. Those 25 become the key-account list.
A list that never changes, so accounts stay on it after they shrink, and concentration risk that rises unnoticed as the list's share climbs. The full guide is key account management.