
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
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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.

women's suit
A two-piece item of clothing for women, consisting of a jacket and a matching skirt or pair of trousers, often worn in a professional context.
Für das Vorstellungsgespräch hat sie sich ein elegantes Kostüm gekauft.
For the job interview, she bought an elegant women's suit.

costume
The complete set of clothing and accessories that actors wear in a play, opera, or film in order to portray their role.
Die Kostüme in diesem historischen Film waren sehr aufwendig gestaltet.
The costumes in this historical film were very elaborately designed.
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.
suit
Both words can refer to a formal combination of a jacket and a matching lower garment. Anzug mainly means the classic combination of a suit jacket and trousers, worn primarily by men; in a professional context, Kostüm usually refers to the corresponding clothing for women, with a skirt or trousers.

Die Frau trug zur Bewerbung ein dunkelblaues Kostüm mit passender Hose.
The woman wore a dark blue women's suit with matching trousers to the interview.
Der Mann trug zur Bewerbung einen dunkelblauen Anzug mit passender Hose.
The man wore a dark blue suit with matching trousers to the interview.