In an IPO, secondary shares (in contrast to primary shares

Primary shares

In an equity offering, primary shares, in contrast to secondary shares, refer to newly issued shares of common stock that are sold to investors...

) refer to existing shares of common stock

Common stock

Common stock is a form of corporate equity ownership, a type of security. It is called "common" to distinguish it from preferred stock. In the event of bankruptcy, common stock investors receive their funds after preferred stock holders, bondholders, creditors, etc...

 that are sold to investors in an offering (see Secondary Market Offering

Secondary Market Offering

A secondary market offering, according to the U.S. Financial Industry Regulatory Authority , is a registered offering of a large block of a security that has been previously issued to the public. The blocks being offered may have been held by large investors or institutions, and proceeds of the...

).
The selling of these secondary shares may be from existing shareholders, or may be newly registered shares from the company. In the case of newly registered shares, the offering is referred to as "dilutive"

Stock dilution

Stock dilution is a general term that results from the issue of additional common shares by a company. This increase in common shares of a stock can result from a secondary market offering, employees exercising stock options, or by conversion of convertible bonds, preferred shares or warrants into...

 (to EPS

Earnings per share

Earnings per share is the amount of earnings per each outstanding share of a company's stock.In the United States, the Financial Accounting Standards Board requires companies' income statements to report EPS for each of the major categories of the income statement: continuing operations,...

) due to the increase in the number of outstanding shares. In the case of selling shares of already registered stock (from an institutional block or from insider shares), the offering is considered "non-dilutive".