A "system call" is a way to call a (privileged) function 
in the operating system kernel. A function is in the
operating system kernel if it needs to run in "kernel mode"
because it needs to do some "privileged" operation" (such
as starting an I/O operation).


Math.sin()             // not a system call
"cat".toUpperCase();   // not a system call

System.out.println()   // this leads to a system call

Scanner in = new Scanner(System.in);
int x = in.nextInt();  // this leads to a system call


OpenFile(data);  // this leads to the NtOpenFile system call
ReafFile(data);  // this leads to the NtReadFile system call


The following are synonyms:
  system call
  trap instruction
  software interrupt