provide evidence for; stand as proof of; show by one's behavior, attitude, or external attributes
"His high fever attested to his illness"
"The buildings in Rome manifest a high level of architectural sophistication"
"This decision demonstrates his sense of fairness"
guarantee payment on; of checks
"The bank refused to certify the bounced check because there were insufficient funds in his account."
declare legally insane
"The judge certified the defendant as legally insane after reviewing all psychiatric evaluations."
To attest to (a fact) as the truth.
"The doctor certified that the patient had recovered from the surgery."
In plain English: To certify means to officially confirm that something is true or meets certain standards.
"The doctor certified that I was fit to return to work after my illness."
Certify comes from the Old French word certefier, which meant to confirm or assure someone. The term traveled into English with this same sense of making a matter certain.