Complexity calculations, assumptions on basic costs
It is generally referred to as "Random Access Machine" or RAM model. This model assumes that accessing any unit of data, irrespective of its position or type, takes the same constant amount of time. Now, what you take as a "unit" of data is important. For all practical purposes, we may consider the primitive data types, integer, float and characters (but not strings) as a unit of data. A single instruction on unit data is considered to take constant amount of time.