eingeschlossen
included
Included in a price or offer; included as part of it.
Bei der Pauschalreise sind die Flüge und das Frühstück im Preis eingeschlossen.
With the package tour, the flights and breakfast are included in the price.

included / locked in
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included
Included in a price or offer; included as part of it.
Bei der Pauschalreise sind die Flüge und das Frühstück im Preis eingeschlossen.
With the package tour, the flights and breakfast are included in the price.

'Eingeschlossen' is the past participle of the separable verb 'einschließen' (to include; to lock in). As an adjective or in stative passive constructions, it is always used with the auxiliary verb 'sein' (to be).
Ich glaube, ich bin im Badezimmer eingeschlossen!
I think I'm locked in the bathroom!
The preposition in the sentence helps you recognize the meaning. 'Im Preis eingeschlossen' means 'included in the price'. 'In einem Raum eingeschlossen' means 'locked in a room'. 'Von etwas eingeschlossen' means 'surrounded by something'.
Das Dessert ist im Preis eingeschlossen.
Dessert is included in the price.
In the meaning 'included', the word is typically used for prices, package tours, and services. It clarifies which services are part of an offer.
Bei diesem Hotelangebot ist die Nutzung des Wellnessbereichs eingeschlossen.
With this hotel offer, use of the wellness area is included.
to contain / abstain
'enthalten' and 'eingeschlossen' can both mean that something is part of an offer, package, or contents. 'enthalten' is a verb and generally describes what is inside something or belongs to it. 'eingeschlossen' is an adjective and fits especially well with prices, offers, or services: it means that you do not have to pay extra for it. Learners may confuse the words because both express the idea of something being included. In practice: a product or text contains something; with a price, something is included.

Beim Hotelpreis ist das Frühstück eingeschlossen.
Breakfast is included in the hotel price.
Der Brief enthält wichtige Informationen für die Anmeldung.
The letter contains important information for the registration.