Delay in processor cycles (MCLK). If your processor runs at a frequency of 1 MHz, then the delay in executing command "__delay_cycles(1000000)" will be, precisely, one second.
* Additional from slau132m.pdf, MSP430 Optimizing 'C/C++' Compiler:
void __delay_cycles(unsigned long);
The __delay_cycles intrinsic inserts code to consume precisely the number of specified cycles with no side effects. The number of cycles delayed must be a compile-time constant.