That depends on whether you talk about individuals, communities, societies, publications,....

Generally speaking, multilingualism refers to the the ability of an individual or a community of speakers to communicate effectively in three or more languages. Such individuals who can speak multiple languages are also identified as polyglots.

To have equal competence and performance in two or more languages.

In my point of view, there is a good reply from Reza Biria

Multilingualism: possession of a set of skills that makes people possible to function, at some level, in more than one language.

Nawel Bengrait

Surely the competence and performance need not be equal in all languages someone speaks/writes? It would be difficult to assess that.

In my view most multlinguals are not polyglots. To be a polyglot is a to have a particular and rare ability to speak (write) a great number of languages (the competence and performance need not be equal).

Åsta: here is (some of) what I've had to say on your question.

What does 'multilingual' mean?: https://beingmultilingual.blogspot.com/2015/10/what-does-multilingual-mean.html

Best

Madalena

Bi/multilingualism means that a person has developed competencies in two or more languages to the extent required by their needs and the needs of the environment in which they live. There are many myths about bi/multilingualism. For example, it is widely thought that in order to be bi/multilingual, a person must have native-like language abilities in both languages. It is also believed that true bi/multilingual will not have an accent in either of their languages. It is not necessary that a person have equal abilities in each language in order to be considered bilingual. In fact, it is rare that bilinguals have an equal ability in each language. There is a continuum of bi/multilingualism that at the high end, might be a person with equal skills in each or his or her two languages. At the other end of the spectrum might be a person whose skills in his or her first language greatly exceed his or her skills in the second language. An example might be a child who is a third-generation immigrant to the United States. He or she may be fluent in English and have some knowledge of Spanish that is used to communicate with grandparents who are not fluent in English.

-Shin, S. (2017). Bilingualism in schools and society: Language, identity & policy. New York: Routledge.

-Baker, C. & Wright, W. (2017). Foundations of Bilingual Education and Bilingualism – 6th Edition. UK: Multilingual Matters.

Thanks for posing this question Adrian!

There are different ways to define this, what about:

- Personal multilingualism (plurilingualism): Ability of a person to use several languages at different levels (see post by Marta Galindo, or CEFR https://www.coe.int/en/web/common-european-framework-reference-languages/level-descriptions ). A transnational mobile resource that a person can use for learning and teaching, and in the way of a social practice (Meier, 2017). A resource through which “relationships and identities are defined, negotiated, and resisted” (Norton and McKinney 2011: 77).

- Societal multilingualism (multilingualism): Co-existence of people who can use different languages in a geographic area or in a group of people (e.g. family, classroom, company, economy).

------

-Meier, G. (2017). The multilingual turn as a critical movement in education: assumptions, challenges and a need for reflection. Applied Linguistics Review, 8(1), 131-161.

-Norton, B., & McKinney, C. (2011). An Identity Approach to Second Language Acquisition In D. Atkinson (Ed.), Alternative Approaches to Second Language Acquisition (pp. 73-94). Abingdon: Routledge.

I would go with definitions focusing on the function of the languages in the persons life-world, like this one from the European Commission (2007): "Multilingualism is understood as the ability of societies, institutions, groups and individuals to engage, on a regular basis, with more than one language in their day-to-day lives" or from Grosjean (2010): "who use two or more languages […] in their everyday lives”. I actually write dedicate some pages on this issue in my PhD thesis :) (open access here on Researchgate)

References:

European Commission (2007), High Level Group on Multilingualism - Final Report, Belgium.

Grosjean, F. (2010), Bilingual: Life and Reality, Harvard University Press, Cambridge.

I personally do not think a person can be called bilingual or multilingual. A person can only use more languages to a lesser or greater extent than a native person of that language. Just as a person is born unique and unique in how to use the language, equally, no matter how well they are trained in that language they cannot use it as completely as a native person in that language. The whole story about bilingualism and plurilingualism and especially the almost forced encouragement in the education, learning and use of several languages has only led to the poor functioning of the native languages and as an extreme of this fact to the disappearance of many languages.