Q:

Which tag(s) from immigration, emigration and migration do we need?

In the Genealogy Conference Room @vervet pointed out that:

As a side note, I noticed the other day when editing a question that
  there are tags for immigration, emigration, and
  migration. It's not really that clear which tag to use as every
  migration includes an immigration and an emigration. By far the most
  commonly used tag is immigration (64) compared to migration (6) and
  emigration (5), which makes me think that the other two should perhaps
  be synonyms of immigration.

If you agree that only one of these is needed with the other two as synonyms, then which should be the master?  Or if you disagree why do you think there should be more than one?

A:

I disagree with this statement:

every migration includes an immigration and an emigration  

Note this definition from Wikipedia:

Emigration is the act of leaving one's native country with the intent
  to settle elsewhere. Conversely, immigration describes the movement of
  persons into one country from another. Both are acts of migration
  across national boundaries.

Especially for large countries such as the United States, Canada, Australia, there can be plenty of things to research about migration which do not involve finding explicit emigration/immigration records.  If we cannot use migration to tag movement within country boundaries, what should we use instead?
The situation is complicated further by the fact that what we call "immigration records" -- e.g. passenger lists required by the US Federal Government to collect data about immigration -- actually contain a great deal of information about the arrival of people who are NOT immigrating -- US Citizens returning home, lists of crew working on the ship, and people coming to the USA who had no intention of making permanent residence here.
I believe there should be three separate tags.

emigration for questions about people leaving their home country  
immigration for questions about people entering a new country
migration for questions about the routes taken by individuals and families because of historical events (the pull & push factors, possible modes of travel, how the geography of an area affects the movement of people, etc.)

In this answer vervet says:

My only issue is that it is often hard to say which tag should be used.

If that is the case, it may be a sign that the question is broad, and could be broken down into multiple questions, or that other tags might be more useful.

A:

I think we should:

use migration as a broad tag to cover all movements of people between and within countries
rename immigration to be called immigration-records and apply it to records relating to the entry of people into a country
rename emigration to be called emigration-records and apply it to records relating to the exit of people from a country

