a talk
These are words for an occasion when someone speaks to an audience on a particular subject.
A talk gives information about a particular subject.
A lecture is a more formal talk given by an expert to a group of people about a subject. Most often, a lecture is when a university teacher talks to a group of students about a subject.
A presentation is a speech in which someone talks about an idea, new product etc., often in a business or education context.
A speech is a formal talk given to a large number of people on a special occasion.
An address is a formal speech, for example, at a conference or an important political occasion.
An oration is a formal public speech about a serious subject, for example about someone who has died. This is a formal word.
A speech in which someone talks about a serious subject in a detail can be called a discourse in formal language. This word can also be used to refer to a piece of writing.
You can use the word eulogy to refer to a speech given at a funeral in which someone talks about and praises the person who has died. A eulogy can also be a piece of writing.
Speaking or public speaking is the activity of giving talks or speeches. These terms are uncountable.