To define the meaning of the eucharist and such things.
Nope. Councils are often called to define doctrine in response to heresies that run rampant. Trent re-affirmed the belief and teaching of the Catholic Church on the Eucharist.
Not a bad answer, but it leaves out a few things. Primarily, major councils such as Trent are called to resolve disputes. They don't define "new doctrines", they either reaffirm or clarify the definitions of previous councils, or define as doctrine what had previously been the consistent, age-old belief of the faithful.
Hence, to say that Trent defined the Eucharist in a particular way is true; to say that Trent defined the Eucharist as something different from what had previously been the consistent belief of the Church is not true. That is the meaning of my remark about "not for the first time". Trent simply reaffirmed what the Church believed since the beginning. Trent was called, and issued its definitions, because a dispute had arisen.