A megachurch is a church with an unusually large membership, who also offer a variety of educational and social activities, usually Protestant or Evangelical.  The Hartford Institute for Religion Research defines a megachurch as any Protestant Christian church having 2,000 or more people in average weekend attendance.    
The concept originated in the mid 19th century, continued into the mid 20th century as a phenomenon, and expanded rapidly through the 1980s and 1990s; it is widely seen across the US in the early 21st century.
The origins of the megachurch movement, with many local congregants who return on a weekly basis, can be traced to the 1800s.   There were large churches earlier in history, but they were considerably rarer. The first evangelical megachurch, the Metropolitan Tabernacle with a 6000-seat auditorium, was inaugurated in 1861 in London by Charles Spurgeon.  In the United States, in 1923, the Angelus Temple was inaugurated in 1923 with a 5,300-seat auditorium in Los Angeles by Aimee Semple McPherson. 
In 2010, the Hartford Institute's database listed more than 1,300 such Protestant churches in the United States; according to that data, approximately 50 churches on the list had average attendance exceeding 10,000, with the highest recorded at 47,000 in average attendance.  On one weekend in November 2015, around one in ten Protestant churchgoers in the US, or about 5 million people, attended service in a megachurch.  3,000 individual Catholic parishes have 2,000 or more attendants for an average Sunday Mass, but they are not called megachurches as that is a specifically Protestant term. 
Globally, these large congregations are a significant development in Protestant Christianity.  In the United States, the phenomenon has more than quadrupled in the past two decades.  It has since spread worldwide. In 2007, five of the ten largest Protestant churches were in South Korea.  The largest megachurch in the world by attendance is South Korea's Yoido Full Gospel Church, an Assemblies of God church, with more than 830,000 members as of 2007.   The largest church auditoriums, Glory Dome, was inaugurated in 2018 with 100,000 seats, in Abuja, Nigeria. 
In 2005, Baptist Pastor Al Sharpton criticized megachurches for focusing on "bedroom morals", statements against same-sex marriage and abortion, by ignoring issues of social justice, such as the immorality of war and the erosion of affirmative action. 
In 2018, American professor Scot McKnight of Northern Baptist Theological Seminary criticized nondenominational megachurches for the weak external accountability relationship of their leaders, by not being members of Christian denomination, further exposing them to abuse of power. 
- List of the largest evangelical churches
- List of the largest evangelical church auditoriums
- List of megachurches in the United States
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