The term is used in a variety of senses, from the very definite arithmetical calculation using an algorithm to the vague heuristics of calculating a strategy in a competition or calculating the chance of a successful relationship between two people.
Multiplying 7 by 8 is a simple algorithmic calculation.
Statistical estimations of the likely election results from opinion polls also involve algorithmic calculations, but give results that are ranges of possibilities rather than exact answers.
Deciding the best way to build a relationship with a member of the opposite sex may also result from a calculation, but is not definite, predictable, nor even clearly defined. This indefinite application of the term gives it a second area of meaning apart from the mathematical senses mentioned above.
To calculate means to ascertain by computing. The English word derives from the Latin calculus, which originally meant a small stone in the gall-bladder (from Latin calx). It also meant a pebble used for calculating, or a small stone used as a counter in an abacus (Latin abacus, Greek abax). The abacus was an instrument used by Greeks and Romans for arithmetic calculations, preceding the slide-rule and the electronic calculator, and consisted of perforated pebbles sliding on an iron bars.