In our Department, all faculty and graduate students attend the colloquium. Eschew jargon or take a moment to explain it. On the other hand, a talk based on a stream of elementary definitions is likely to be dull. A picture is worth a thousand words. You only have fifty minutes to communicate, and so it may be better to inform our intuition graphically, rather than to give a precise explanation of every concept.
Audiences love context.
If Gauss thought about the problem, tell us. Exhaustive lists of names and dates are usually of interest only to the specialists. Show us an example. It is remarkable how effective this can be in reaching out to people who might otherwise not know what you are talking about. Grab our interest with a special case.