Is it an opioid or is it not? The FDA site says it's a synthetic opioid, which means it's lab created like fentanyl, correct? It also warns against using tramodol with certain Anti depressants. Could some one please explain what "Serotonin Syndrome" is as it is associated with Tramodol? Both sites strongly advise about its' addictive nature. After reading this information, is it safe to say, Tramodol is an opioid and it is addictive?

This first part is copied directly from the Drugs.com medication information page.

"Ultracet is used to treat moderate to severe pain for a period of five days or less. It contains two pain-relieving agents. Tramadol, known technically as an opioid analgesic, is a narcotic pain reliever. Acetaminophen is the active ingredient in the over-the-counter pain remedy Tylenol"

This link is through drugs.com also but the information is from the FDA.
https://www.drugs.com/pro/ultracet.html

The FDA one also states,
"Tramadol is a centrally acting synthetic opioid analgesic. Although its mode of action is not completely understood, from animal tests, at least two complementary mechanisms appear applicable: binding of parent and M1 metabolite to µ-opioid receptors and weak inhibition of reuptake of norepinephrine and serotonin"