Regulation A is the SEC framework allowing companies to make public-like securities offerings up to $75 million annually with less burden than a full IPO. Often called Reg A+ following the 2015 expansion under the JOBS Act, it is structured in two tiers (Tier 1 up to $20M with state-level coordination, Tier 2 up to $75M with federal preemption of state law). It is sometimes called a "mini-IPO" because shares can be freely traded post-offering and the company can market the offering publicly. It is the regulatory layer between Reg CF crowdfunding (smaller, less complex) and full IPO (larger, much more complex), occupying a middle space that few companies actually use but that fits specific situations well.
The two tiers: