NOTE: I am giving my best guesses and I am no expert, but hopefully this comment can serve as some kind of jumping-off point.
I don't really know if a good one-sentence definition, and I usually tend to think of it in terms of historical movements, especially classical liberalism, which Wikipedia describes as "a branch of liberalism that advocates civil liberties under the rule of law with an emphasis on economic freedom." So if you want a textbook definition, I guess you could do worse than that, but it doesn't tell the whole story.
I tend to use the word "liberalism" to describe worldviews that can be traced back primarily to classical liberalism or its offshoots like social liberalism. This describes most of the major political parties in most Western countries, including almost all Democrats and Republicans in the US, almost all Conservatives, Liberals and NDP members in Canada, and so on. The term also applies to libertarians, and there's an argument to be made that it applies to many forms of anarchism as well.
People like Karl Marx and Friedrich Engels wrote arguments that were in dialogue with 19th-century liberals, and a lot of what we call Marxism and socialism developed in response to liberalism. Liberalism places a lot more emphasis on individuals as the primary unit of analysis for understanding society and on laws that prevent one individual from harassing another. Which are not terrible ideas, but it ignores e.g. class, racial and gender hierarchies, systemic oppression, existing power structures. Maybe "ignores" is the wrong word -- it's more like liberalism, on its own, is incapable of meaningfully discussing them at all. It's a blind spot.
Example: When the George Floyd protests were in the news, I had a lot of well-meaning older relatives who posted things on Facebook about how you shouldn't judge people by the colour of their skin and we should all just treat each other as individuals. Which is not totally wrong, but it's incomplete as an explanation of racism and not really relevant to the murder of George Floyd. The reason black people are disproportionately killed by police isn't just because individual police officers are racist and we just have to teach them to not be racist. It has to do with the history of policing in the United States, and with policies that have been selected specifically to put black communities at a disadvantage in order to protect the existing distribution of wealth.
Leftists tend to talk derisively about liberals and liberalism, partly having to do with the fact that capitalism is historically very intertwined with liberalism, but I think that there's room for nuance and that there are some good things to come out of liberalism. The idea that people should be free to pursue what they want to do as long as they don't oppress others is not a bad idea, even if it's incomplete. I mentioned that Marxism and socialism developed partly in response to liberalism, and it wasn't 100% disagreement (e.g. Karl Marx acknowledged that capitalism and liberalism were still preferable to feudalism and theocracy). I think our current views on the importance of consent in sexual activity stem from liberalism, and I certainly wouldn't want to give up on those.
Again: This is my flawed understanding and you should take everything I'm saying with a huge grain of salt.
NOTE: I am giving my best guesses and I am no expert, but hopefully this comment can serve as some kind of jumping-off point.
I don't really know if a good one-sentence definition, and I usually tend to think of it in terms of historical movements, especially classical liberalism, which Wikipedia describes as "a branch of liberalism that advocates civil liberties under the rule of law with an emphasis on economic freedom." So if you want a textbook definition, I guess you could do worse than that, but it doesn't tell the whole story.
I tend to use the word "liberalism" to describe worldviews that can be traced back primarily to classical liberalism or its offshoots like social liberalism. This describes most of the major political parties in most Western countries, including almost all Democrats and Republicans in the US, almost all Conservatives, Liberals and NDP members in Canada, and so on. The term also applies to libertarians, and there's an argument to be made that it applies to many forms of anarchism as well.
People like Karl Marx and Friedrich Engels wrote arguments that were in dialogue with 19th-century liberals, and a lot of what we call Marxism and socialism developed in response to liberalism. Liberalism places a lot more emphasis on individuals as the primary unit of analysis for understanding society and on laws that prevent one individual from harassing another. Which are not terrible ideas, but it ignores e.g. class, racial and gender hierarchies, systemic oppression, existing power structures. Maybe "ignores" is the wrong word -- it's more like liberalism, on its own, is incapable of meaningfully discussing them at all. It's a blind spot.
Example: When the George Floyd protests were in the news, I had a lot of well-meaning older relatives who posted things on Facebook about how you shouldn't judge people by the colour of their skin and we should all just treat each other as individuals. Which is not totally wrong, but it's incomplete as an explanation of racism and not really relevant to the murder of George Floyd. The reason black people are disproportionately killed by police isn't just because individual police officers are racist and we just have to teach them to not be racist. It has to do with the history of policing in the United States, and with policies that have been selected specifically to put black communities at a disadvantage in order to protect the existing distribution of wealth.
Leftists tend to talk derisively about liberals and liberalism, partly having to do with the fact that capitalism is historically very intertwined with liberalism, but I think that there's room for nuance and that there are some good things to come out of liberalism. The idea that people should be free to pursue what they want to do as long as they don't oppress others is not a bad idea, even if it's incomplete. I mentioned that Marxism and socialism developed partly in response to liberalism, and it wasn't 100% disagreement (e.g. Karl Marx acknowledged that capitalism and liberalism were still preferable to feudalism and theocracy). I think our current views on the importance of consent in sexual activity stem from liberalism, and I certainly wouldn't want to give up on those.
Again: This is my flawed understanding and you should take everything I'm saying with a huge grain of salt.