What is Bulge Bracket?

Legal Definition
The bulge bracket comprises the world's largest and most profitable multi-national investment banks whose investment banking clients are usually large corporations, institutions, and governments. They usually provide both advisory and financing banking services, as well as the sales, market making, and research on a broad array of financial products including equities, credit, rates, commodities, and their derivatives. They are also heavily involved in the invention of new financial products, such as mortgage-backed securities in the 1980s, credit default swaps in the 1990s, and today, carbon emission trading and insurance-linked products. Bulge bracket firms are usually primary dealers in US treasury securities. Bulge bracket banks are also global in the sense that they have a strong presence in all three of the world's major regions: The Americas, EMEA, and Asia-Pacific.

The name comes from the way investment banks are listed on the "tombstone", or public notification of a financial transaction or deal. The bank responsible for control of allocation of securities to investors, known as the bookrunning manager is listed above the others and on the cover of the prospectus. The font size of the name of this bank, or banks if there are co-bookrunning managers, is larger and it may "bulge" out.

There is often debate over which banks are considered to belong to the bulge bracket. Membership implies prestige, but there are no precise criteria for inclusion, and financial power is transient. Various rankings are often cited, such as Thomson Reuters League Tables, Bloomberg 20, or other league tables.
-- Wikipedia
Legal Definition
The small group (i.e., less than half a dozen) of security firms, investment banks, and banks that control the largest share of global corporate finance and securities underwriting business. Bulge bracket firms are often sought out by institutional clients as a result of their ability to arrange sophisticated transactions and distribute securities.