Dic: by (from what something shows or says; according to a standard, rule, law)

9△ from what something shows or says; according to something.

• By my watch it is two o'clock.
• I could tell by the look on her face that something terrible had happened.
• By law, you are a child until you are 18.

..... OALD

7. PREP△ If something must be done by law, it happens according to the law. If something is the case by particular standards, it is the case according to the standards.

• Pharmacists are required by law to give the medicine prescribed by the doctor.
• evening wear that was discreet by his standards.

....... Cobuild

CF.

• By this strict definition, few markets in the U.S. economy are perfectly competitive.