Single assignment

Single assignment means that each variable in a computer program can only have one value at any given time. This means that if a variable has a value and you try to give it another value, the program won't let you do it. It's like a box that can hold only one object at once. If you put something in the box and then you try to put something else in the box, the first thing will get replaced by the second thing. That's single assignment!