5PM. There's a mass exodus out of the city as people depart for home. What word describes the migration in the other direction -- A mass arrival of people? As far as I know, there isn't a word like "introdus".
Perhaps a word like "intrution"?
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Though I have not been able to find this anywhere quoted as an antonym, I feel fairly sure the modern word, in current use would be influx.
Oxford Dictionaries provides its literal meaning to do with an inflow of water into a lake, secondary significance. Its first meaning, according to OD is: an arrival or entry of large numbers of people or things. e.g 'a massive influx of tourists'
Its etymology is late 16th century (denoting an inflow of liquid, gas, or light) from Latin influxus, from influere flow in (see INFLUENCE).
Describing the context will be helpful. Are you talking about something that is happening routinely/repeatedly (e.g. people coming back from holidays) or some sudden/one-time event (e.g. refugees)? If it's the former "influx" as mentioned above would be adequate. In the latter case I would, perhaps, use "a flood of new arrivals" if dramatic effect is sought.