I'm studying basic algebra and I'm a bit confuse about degree of equations. I read quadratic equation are equations of the second degree, meaning it contains at least one term that is squared. My question is if that squared term also can be something like $2xy$ (because the degree of the term is also $2$ (the sum of exponents of each variable): example $2xy + x = 12$ or can only be single variables with exponent $x^2, y^2, \dotsc$.
As said in the comments, when dealing with more than one variable you simply add the exponents of each.
For example, the degree of $xy$ is $1+1=2$ (since $x$ and $y$ are raised to the $1$st power.) Similarly, the degree of $x^2y$ is $2+1=3$, and the degree of $xyz$ is $1+1+1=3$.