An Algorithm
An algorithm is a well-ordered collection of unambiguous and
effectively computable operations that when executed produces
a result and halts in a finite amount of time.
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well ordered
it is always obvious what instruction is done next
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unambiguous
an operation that can be carried out without further
explanation or simplification
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effectively computable
an operation is doable, that is, it is not something like
"write out the last digit of pi" or "give a list of all prime numbers"
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that produces a result
some form of output that can be checked - it may be the answer
or it may be some indication of error
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and halts in a finite amount of time
you do not want an infinite (infinitely repeating) loop,
a set of instructions that never stops running.