The definition, in plain terms
A sermon is public teaching from Scripture, addressed to a gathered congregation, with the goal of changed hearts and lives — not just transferred information. That last part is what separates preaching from lecturing: a lecture aims at understanding, while a sermon aims at response.
Most Christian traditions treat the sermon as a central act of worship. In many Protestant churches it is the longest single element of the service, commonly 25 to 45 minutes.