Matéria vs. disciplina vs. assunto
When discussing school and academics in Portuguese, the words matéria, disciplina, and assunto all relate to the English concept of subject, but they operate at different levels of hierarchy. Generally, disciplina is the formal course name, matéria is the content or syllabus to be studied (or colloquially the class name in Brazil), and assunto is the specific topic within that content.
Matéria
A2In Brazil, matéria is the most common colloquial term to refer to a specific school subject, such as Math or Geography. However, in Portugal (and formally in Brazil), it refers specifically to the syllabus, the content covered in a class, or the material required for an exam, rather than the name of the course itself.
Minha matéria favorita na escola sempre foi História.
(My favorite subject at school was always History.)
Você estudou toda a matéria para o teste de amanhã?
(Did you study all the material for tomorrow's test?)
O professor atrasou a matéria porque a turma teve dúvidas.
(The teacher fell behind on the syllabus because the class had questions.)
Disciplina
B1This is the formal and administrative term for a specific field of study or course. In Portugal, disciplina is the standard word for school subjects (like Physics or Biology) in everyday speech. In Brazil, it is generally used in university contexts or formal documentation, while students prefer matéria for K-12 education.
Matemática é a disciplina mais difícil do currículo.
(Mathematics is the most difficult subject in the curriculum.)
Preciso me inscrever em cinco disciplinas neste semestre.
(I need to enroll in five courses this semester.)
Os alunos devem manter o silêncio durante essa disciplina.
(Students must maintain silence during this class.)
Assunto
B1This word translates to topic, issue, or subject matter. It refers to a specific point or theme being discussed within a broader disciplina or matéria. For example, if the class is Biology, the assunto might be Photosynthesis.
O assunto da aula de hoje é verbos irregulares.
(The topic of today's class is irregular verbs.)
Vamos mudar de assunto e falar sobre a prova.
(Let's change the subject and talk about the exam.)
Esse assunto não cai no vestibular.
(This topic is not covered in the entrance exam.)
Summary
To summarize: use disciplina for the formal name of the course (standard in Portugal, academic in Brazil). Use matéria for the content/syllabus you need to learn (or informally for the class name in Brazil). Use assunto for the specific topic or theme discussed within that class.