das Kostüm
costume
A disguise or outfit worn for a specific occasion such as Carnival, a themed party, or a theater performance.
An Halloween tragen viele Kinder gruselige Kostüme.
On Halloween, many children wear scary costumes.

costume / suit
Loading your saved state.
costume
A disguise or outfit worn for a specific occasion such as Carnival, a themed party, or a theater performance.
An Halloween tragen viele Kinder gruselige Kostüme.
On Halloween, many children wear scary costumes.

Unlike in English, where 'costume' usually means a disguise, 'das Kostüm' in German can also refer to an elegant trouser suit or a jacket-and-skirt ensemble for women. This meaning is common in business life.
Für das Bewerbungsgespräch hat sie ein neues Kostüm getragen.
For the job interview, she wore a new women's suit.
The meaning of 'Kostüm' (costume/suit) depends strongly on the occasion. At parties (Carnival, Halloween), it is a costume. At work, it is a formal outfit for women. In theater or film, it is the actors' clothing.
Mein Sohn will zu Karneval ein Piraten-Kostüm.
My son wants a pirate costume for Carnival.
When 'Kostüm' (costume) means a disguise, it is often used in connection with festivals such as 'Karneval' (Carnival), 'Fasching' (Carnival), or 'Halloween'.
Was für ein Kostüm ziehst du zur Faschingsparty an?
What kind of costume are you wearing to the Fasching party?
In a professional theater and film context, there is the job 'Kostümbildner/in' (costume designer). This person is responsible for the design and production of the costumes.
Die Kostümbildnerin hat für ihre Arbeit an dem Film einen Preis gewonnen.
The costume designer won an award for her work on the film.
mask
'die Maske' and 'das Kostüm' can both be part of a disguise, but they are not the same thing. A 'Maske' covers the face or part of the face. A 'Kostüm' is the clothing used to represent a role or character, for example at Carnival or in the theater. They can be confused because people often wear both together. If it is only about the face, 'Maske' is correct. If it is about the whole disguise or outfit, 'Kostüm' fits better.

Für die Schulfeier hat sie ein Piratenkostüm vorbereitet.
She prepared a pirate costume for the school party.
Mein Nachbar trägt morgen eine schwarze Maske zum Karneval.
My neighbor is wearing a black mask to Carnival tomorrow.