Meaning of “matter” - Learner’s Dictionary

matter

noun uk us /ˈmætər/
SUBJECT [ C ]

B2 a subject or situation that you need to think about, discuss, or deal with:

I've been thinking about this matter for a long time.
He denied any knowledge of the matter.
To make matters worse, our car broke down!
SUBSTANCE [ U ]

the physical substances that exist in the universe

TYPE OF THING [ U ]

a particular type of substance or thing:

vegetable matter
printed matter
what's the matter

A2 used to ask or talk about the reason for a problem:

What's the matter with your leg?
I can't help you if you won't tell me what the matter is.
there's something/nothing the matter

A2 used to say that there is/is not a problem:

There's something the matter with the washing machine.
a matter of days/weeks/feet, etc

used in expressions describing how small a period of time or an amount is:

The aircraft missed each other by a matter of feet.
a matter of confidence/luck/waiting, etc

If something is a matter of confidence/luck/waiting, etc, that is what you need for it to happen:

Learning languages is just a matter of hard work.
no matter how/what/when, etc

B2 used to emphasize that something cannot be changed:

I never manage to lose any weight, no matter how hard I try.
as a matter of fact

B2 used to emphasize that something is true, especially when it is surprising:

As a matter of fact, I used to live next door to him.
a matter of course

If something happens as a matter of course, it always happens as part of the normal process or system:

Babies were tested for the disease as a matter of course.
a matter of life and/or death

a serious situation where people could die:

Getting water to these people is a matter of life and death.
it's only a matter of time

If you say that it is only a matter of time before something happens, you are sure it will happen but you do not know when.

(Definition of “matter noun” from the Cambridge Learner’s Dictionary © Cambridge University Press)