Ethnocentrism, a term coined by social evolutionist William G. Sumner in 1906, is generally defined as "the belief in one's own ethnic group's superiority".
Ethnocentrism is generally defined as "the belief that one's own native culture is the most natural or superior way of understanding the world". It's the tendency to look at the world primarily from the perspective of one's own culture.
"This tendency to view the Other from the perspective of one's own is often accompanied 1) by feelings of dislike for other groups or cultures, 2) by judging other cultures solely by the values and standards of one's own culture, 3) judge other groups relative to his or her own particular ethnic group or culture, especially with concern to language, behavior, customs, and heritage."
It's easy to see that people around the world are divided by geographical boundaries and speak different languages. But, it's easy to overlook that people are divided by physical boundaries (a sense); as a result, Deaf people speak a different language in a different modality. Deaf people become a cultural-lingual group and don't consider themselves as a disability group defined by hearing people. Disability is a social construction defined within the phonocentric society. Hearing people don't realize that they do have a 'hearing culture' recognized and defined by Deaf people.
This leads to a number of -isms within the hearing world: phonocentrism, audism, ethnocentrism, linguicism, and so on. Many hearing people don't realize that they've internalized these -isms over thousands of years. Ethnocentrism occurs everywhere and everyday.
When studying ASL/Deaf culture and language, hearing students learn more about their own 'hearing' culture, thus developing more cultural relativism and reducing ethnocentrism. With this in mind, when studying, it is important to examine the bases for one's beliefs regarding another culture.
You may also be interested in: phonocentrism.