A secondary offering is the sale of a large block of previously-issued, privately-held stock, which actually requires registration with the SEC, but does not raise capital for the company which issued the shares originally.
A secondary offering is a non-dilutive sale of existing shares which were previously held by one, or a few, investors. The proceeds of the sale go to the sellers of the shares and not to the company which issued the shares.
When a block of securities big enough is sold in this manner, it does require registration with the SEC. This term is often used mistakenly in the place of a Follow-On offering, which is a new issue of shares from the company which initially issued the stock.
The latter offering is dilutive and requires underwriting and many of the same regulations apply as with an IPO (initial public offering). A secondary offering is made in the secondary market.
While it is not dilutive, such a large block does increase the liquidity of the shares outstanding and will most likely affect trading volume in the stock.
What is Secondary Market?
What does Over-the-Counter (OTC) mean?