There are myriad terms we use to describe teaching in a language, and they are often used interchangeably as they spread from one context to another. One of these terms is CLIL; it was developed in a certain context, to describe a type of programme, and has since become used to describe a wider sets of related but distinct practices. Here is an overview of terms and methods currently in use.
EMI: English-medium education
This describes any type of teaching in which the delivery language is English. It can describe classes with only first-language English speakers, but is more commonly used to describe classes in which non-English speakers are being taught in English.
CBI: Content-based instruction
This describes teaching which focuses on the delivery of content through another language. It is commonly used in immersion programmes, where the students learn a new language by being taught in that language. It differs from CBI only in that the language of instruction can be any language, not only English.
CLIL: Content and Language Integrated Learning
This describes teaching that has features of both CBI/EMI and features of more classical language teaching. The expectation in CLIL teaching is that the students will learn by being taught in the language, but that there will also be an overt focus on language development, by means of explicit focus on form and/or vocabulary development. In my experience, a lot of what schools call CLIL is actually CBI/EMI; they are teaching through English, but not about English.
Does it really matter what we call it?
I think that there needs to be a clear distinction between EMI and CBI, which are closely related, and CLIL, which should be distinct from the former. EMI and CBI are immersion-delivery methods. They expect that the content, in part or completely, should be taught through the target language, and that this will provide the basis for language development. There is no proscribed methodology or pedagogy, simply the expectation of content delivery through the L2. CLIL, on the other hand, is a methodology that relies on content-based instruction, with enhanced focus on language. It is based in CBI/EMI, but extends to include language teaching as well. When looking at the research on pure immersion, it is clear that immersion only does not provide for full development of accuracy and academic fluency in a language, especially for writing. Integration of content-related focus on language provides scaffolding for learners that allows for better development in certain areas of language development.
In essence, any programme that teaches students who are language-learners can be designated as CLIL if the teachers are attending to the development of necessary grammatical skills and vocabulary development as linked to the content. It sounds simple, but it actually quite challenging to achieve, which is why many CLIL programmes end up functionally as EMI programmes. In order to be a successful CLIL teacher, you need to be able to develop language across your teaching. This means understanding the language-related demands of any topic/unit in terms of language functions, necessary grammatical structures, and the three tiers of vocabulary. It's like expecting a mind-meld between a subject-teacher and an English teacher, both bringing their own knowledge and planning skills to the table. Being a proficient CLIL teacher requires a deep level of knowledge both about the subject you teach and the language you are teaching in. In some schools, there is a structure in place to have English specialists provide support for CLIL subject teachers, which can be a very successful model if a solid approaching to co-planning is adopted. Too often, subject teachers are left on their own, and end up delivering their course as an EMI course, with a simple immersion method.
Genesee, F., & Hamayan, E. (2016). CLIL in Context: Practical guidance for educators. Cambridge: Cambridge University Press.
Lyster, R. (2007). Learning and teaching languages through content: A counterbalanced approach. Amsterdam: John Benjamins.
Mehisto, P. (2017). CLIL Essentials for Secondary School Teachers. Cambridge: Cambridge University Press.