A logarithm is a number that stands for another number in a special maths system. The useful trick is this: if you want to multiply two hard numbers, you can add their logarithms instead, which is much easier. Before cheap calculators, people used printed logarithm tables for this. At school it is usually written log, and sometimes ln for a special kind. It is a classroom and science word, not an everyday one. A “logarithmic scale” (earthquakes, sound, some graphs) uses this idea: each step is a jump in size, not a small equal step. The word is long, but the job of the word is only to count how far a number sits from a starting value when you grow it by the same ratio again and again.