I'm not sure there is a good definition that you can simply "quote".
For example, what does "TCP/IP" mean? Well, it stands for "transmission control protocol / internet protocol". That's completely correct, but at the same time, completely unhelpful. If you didn't know what TCP/IP was to begin with, expanding the acronym won't help you.
A helpful definition is this: Hackers exploit certain types of bugs in software to break into computer; software security is the science of getting rid of those bugs.
For example, a common bug hackers exploit is the "buffer-overflow". They happen because reserves a certain amount of memory to hold something, but hackers provide more than the programmer expected. For example, in a database holding the username, the programmer might assume that nobody would every have a name with more than 1000 letters. That's true, they won't -- but that won't stop hackers from attempting to enter false names that long, and when they do, buffers overflow, hackers overwrite other parts of memory and take control of the system.
We know why buffer-overflows are so common. Take the C/C++ language, for example. Back in the 1970s, frequently reused code like "strcpy()" and "sprint()" were developed that copy buffers in memory without double checking the length. Therefore, hackers simply search software for uses of these code snippets and find these bugs. Software security does this before the hacker, finding all uses of "strcpy()" and removing them.