What is Culture, exactly?

It sounds like an idiotic question but every time I come across it in my book I'm not really sure what "culture" they are referring to. Sometimes it clearly means race or ethnic background, like in the word multicultural, but other times its meaning is less specific. It seems like one of those nebulous words that are tossed around like diversity. It's safe because it can include everything or just refer to race and sometimes its hard to tell. If it seems to refer only to race it is sure to include every other "culture," just in case. I don't know whether to press my teacher on this one. I wonder if "culture" is meant to only mean a person's background as in where they are from or can the meaning be more complex to imply that a person's culture could be a different life perspective, social class, generation, sexual orientation or gender identity, and I'm sure the list goes on. I'd like to think culture has a broader meaning but maybe our society or certain institutions have narrowed the meaning so that most people jump to "race" when they hear "culture." And if that were true, what a problem that would be, a great way to enforce stereotyping and a lack of understanding for the myriad of differences we all have.