Computational complexity theory
This is often the complexity for problems that involve looking things up, like finding a word in a dictionary. If the dictionary is twice as big, it contains twice as many words as the original to compare to. Looking something up will take only one step more. The algorithm to do lookups is simple. The word in the middle of the dictionary will be either before or after the term that needs to be looked up, if the words do not match. If it is before, the term needs to be in the second half of the dictionary. If it is after the word, it needs to be in the first half. That way, the problem space is halved with every step, until the word or definition is found. This is generally known as logarithmic complexity