There are a variety of ways to say "by one order less." Some synonyms include "one level lower," "one tier below," "one grade lesser," "one magnitude smaller," "less by a power of ten," or simply "one step down." These phrases can be used in a variety of contexts, such as when discussing scientific measurements or numerical values. Choosing the right synonym for "by one order less" can help to convey the intended meaning clearly and concisely, so it's important to select the one that best fits the specific situation.