oral presentation intended to present information or teach people about a particular subject
A lecture is when someone talks to a group of people about something interesting. It is like a lesson. At college or university students go to lectures to learn things. The person who is teaching (talking to the group) is called the lecturer or sometimes professor. The lecturer may accept questions from the audience.
Sometimes we say that someone was "given a lecture". This means that he was told off for doing something wrong.