Ageism was coined as a term and concept as a reaction against discriminatory employment practices that resulted in things like being unable to secure a new job at a commensurate level when over the age of 50 and being forced to retire when reaching the ages of 60 to 65. Ageism is discriminatory and/or prejudicial behavior and policies toward any individual based on their age; this now includes all age groups: children, teens, young adults, people over 50, and the elderly.
I have to agree with the definition of homophobia. The problem with homophobia is that, sometimes, it is based upon assumptions. Ageism and apartheid can both be "seen" in the characteristics of the one being oppressed or prejudiced against. People who are homophobic simply think that a person is gay and become afraid (problems lies in the fact that assumptions are dangerous--a person may not be gay, but the homophobic person may think that they are).
I would highlight the Rwandan genocide as a particularly tragic example of ethnonational conflicts, as it occurred between to ethnic groups in Rwanda: the Tutsis and the Hutus. The way in which the conflict enabled these groups to express their tribal identity in a very violent and oppressive way is something that Rwanda is still trying to recover from. Ethnonational conflicts are therefore when one group within a nation prizes their tribal or ethnic identity above their national identity, with normally very bloody results.
Ethno-national conflict between ethnic groups for control over a territory, resource, government, etc. They can include uprisings by separatist groups, civil wars or power struggles, or other forms of fighting amongst distinct groups. Some recent examples would include struggles between Catholics and Protestants in Northern Ireland, between Kyrgyz and Uzbek factions in Kyrgyzstan, the Rwandan civil war and genocide, or even the civil war in Iraq.
Homophobia is a little harder to define. It's basically fear or hatred of gay people. But it is (like racism) sort of hard to tell where it begins. It is also hard to know whether a person should be called homophobic if they have serious religious/moral misgivings about homosexuality but they treat gay people as their equals in real life. But basically, it's like racism except based on sexual orientation.
"Ageism" is prejudice against a person because the person is assumed, for instance, to be too old to perform a task properly. Or such prejudice might involve discrimination against a person because that person no longer looks young. Any prejudice or discrimination against a person based automatically and without firm evidence on that person's advanced age might be called "ageism." For instance, if I assume that a person is too old to drive a car simply because of the person's age, and if I do not even bother to test that person's actual abilities, I might reasonably be accused of "ageism."
Apartheid was the official governmental racial system in South Africa from the late 1940s until it was abolished in 1993. Under the apartheid system, the rights of non-whites were restricted by the white majority in areas that included education, medical care, citizenship and other public services.