A medium of instruction is the language used by a teacher to teach students. Simply put, it is a means of conveying information to students. Such a medium could be the official language in the country or it could be the native mother tongue of the students.
In an English speaking country like the United States, the medium of language used by teachers to teach students is English. Any immigrant that comes to the United States from another country would have to learn the language so as to be able to join other students in class. In some types of education, such as multilingual or bilingual education, the teachers may use more than one medium of instruction to teach the students.
Deciding which language to use for instruction is important because it can affect the way the students learn. Someone who grew up speaking French or Russian might not adapt easily to the concept of learning in English. In situations like that, a combination of the mother tongue and the chosen medium of instruction will work better, until the person is more confident in the new language. For instance, if it is a bilingual type of education and the students are a mixture of Spanish and English, the medium of instruction will not be only English, but rather a mixture of Spanish and English. In multilingual education, the language of instruction will not be just one language, it will also include two or more other languages.
The mother tongue could be used to explain the methodology and issues behind problems before switching to the medium. It is important to allow students to grow accustomed to the concept of learning in the chosen medium. This is because the more exposed a student is to the new language, the more he or she will learn. If the student keeps referring back to the mother tongue, he or she will likely not have the motivation to learn the new language. An immigrant to a new country may not understand the message conveyed through a medium of instruction because he or she cannot understand the language; but over time, with practice, he or she will start to grasp the fundamentals of the new language.
With globalization and the ease with which people interact with others across international borders, the issue of what medium of instruction to use comes up regularly. This is especially the case for new immigrants who do not understand the language of the country. Performance in some subjects like science might be affected by the use of a medium that is unfamiliar to the students because such subjects demand a higher degree of abstract thinking, language proficiency, and the mastery of scientific terminology.