Definition by Wonder Me!:
Full agonists activate maximum effects when binding to receptors.
Partial agonists activate less than maximum effects when binding to receptors

Clinical example:
Methadone, like heroin, is the full agonist for opioid effects
Buprenorphine as the partial agonist for opioid effects

Why it works?:

"For people who are not addicted to or dependent on opioids, the effects of partial (buprenorphine) and full (methadone) agonists are indistinguishable. However, at a certain point, the increasing effects of partial agonists reach maximum levels. For this reason, people who are dependent on high doses of opioids are better suited to treatment with a full agonist, such as methadone." (1)