An SEC form similar to the Schedule 13D used to report a party's ownership of stock that is over 5% of the company. Schedule 13G is shorter and requires less information from the filing party. Ownership of over 5% in a publicly-traded stock is considered to be significant ownership, and therefore must be reported to the public.

Investopedia Says:
To be able to file a 13G instead of a 13D, the party must own between 5% and 20% in the company. It must also be clearly understood that the party acquiring the stake in the company is only a passive investor, and does not intend to exert control. If these criteria are not met, and if the size in the stake exceeds 20%, a 13D must be filed.