According to the Merriam-Webster Dictionary: Major party: a political party having electoral strength sufficient to permit it to win control of a government usually with comparative regularity and when defeated to constitute the principal opposition to the party in power.
Typically, major parties have the most donors, best-organized support networks and excellent funding for elections.
Their candidates for political positions are closely watched since they have the highest chance of being elected to office because of the high membership, recognition and donations that these parties are able to generate.
Two major parties can lead to a two-party system.
For example, the FAQ posted on the website of Michigan Independent Citizens Redistricting Commission says that "Of the 13 commissioners, four affiliate with the Democratic Party, four affiliate with the Republican Party, and five do not affiliate with either major political party.