Liberalism has its roots in the Age of Enlightenment, late 1600s-late 1700s. The father of liberalism, John Locke—an English philosopher—thought that reason and tolerance were quintessential to human nature and that the people are the ones who give the government its power. He believed strongly that life and liberty were natural rights for every human.
Thomas Jefferson closely followed the ideas of European philosophers of this era and incorporated some of these ideas into the Declaration of Independence. James Madison, another founding father, incorporated similar ideas into the United States Constitution.
The political/ideological movement is called liberalism because of its base in liberty and equality before the law. Documentation from all the way back in 1781 shows it being used to mean “free from narrow prejudice.” Today it can be used to mean anything left of center or anyone who believes that the people give the government its power and the government should regulate the economy.
The American Great Depression, 1929-1941, sparked a spike in liberal ideologies. This period saw the creation of the Social Security Administration and an overall increase in government involvement in the wellbeing of its citizens. Into the 1960s, liberalism focused more on civil rights, environmental activism, and the expansion of welfare and other government programs.
Today liberalism focuses on reproductive rights, LGBTQ rights, support of individuals over corporations, and the opposition of tax cuts for the rich.