I agree with John. I often hear the term conference used for a meeting where a speaker phone is used to have many parties in different locations. This is a 'conference call'. It is when this term is shortened to 'conference' that it is a synonym for meeting. There is a big difference between a meeting and a convention / conference - the latter is a collection of professional speakers, or demonstrators that are maybe in the same industry or hobby. They all come together at a venue, and offer talks, sessions on a schedule, or at their 'booth' (an allocated space within the venue). The visitors to the convention can walk around all the booths, an watch demonstrations, or talk to the vendors. It is a bit like a temporary market, on a special subject.

Meeting implies a relatively small number of people gathering for a relatively brief amount of time to discuss a particular issue or participate in a specific activity. You may have a meeting with colleagues at work to discuss a business plan, or a meeting with people that are all interested in the same hobby. Conference and convention are larger gatherings that may last the entire day or be several days long. For example, I recently went to a conference for my profession. Approximately 600 people attended from many different companies, and it lasted 3 days. It is common substitute the word meeting for conference, but it would be unusual to call a brief meeting at work a conference. Also, conference typically refers a professional gathering whereas convention may refer to a hobby. As you can see, it's not always clear but in my experience this is how these words are used.