
das Stadium
stage
A particular section or phase in a development, process, or disease.
Die Verhandlungen sind in ein kritisches Stadium getreten.
The negotiations have entered a critical stage.
stage / stadium
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stage
A particular section or phase in a development, process, or disease.
Die Verhandlungen sind in ein kritisches Stadium getreten.
The negotiations have entered a critical stage.

stadium
A large, usually open sports facility with stands for spectators. Today, the word 'Stadion' (stadium) is used almost exclusively.
Das antike Stadium von Olympia bot Platz für tausende Zuschauer.
The ancient stadium of Olympia had room for thousands of spectators.
In the meaning 'phase', 'Stadium' (stage) is usually used with the preposition 'in' and the dative. Typical verbs are 'sein' (to be), 'sich befinden' (to be located/to be), 'eintreten' (to enter), or 'erreichen' (to reach).
Das Projekt befindet sich noch in einem frühen Stadium.
The project is still in an early stage.
'Stadium' (stage) is often combined with adjectives that describe a phase in a process, for example 'frühes Stadium' (early stage), 'fortgeschrittenes Stadium' (advanced stage), or 'letztes Stadium' (final stage).
Die Krankheit wurde zum Glück in einem sehr frühen Stadium entdeckt.
Fortunately, the disease was discovered at a very early stage.
This meaning of 'Stadium' (stage) is found in contexts that describe a process or development. These include projects, negotiations, diseases, or biological developments.
Die Raupe ist ein Stadium in der Entwicklung eines Schmetterlings.
The caterpillar is a stage in the development of a butterfly.
In the meaning of a sports facility, 'Stadium' (stadium) refers almost exclusively to historical or ancient structures. This word is not used for a modern sports arena.
Bei der Führung durch Rom besichtigten wir ein altes römisches Stadium.
During the tour through Rome, we visited an old Roman stadium.
phase
Phase emphasizes the period and its characteristic progression. Stadium refers more to the developmental state that has been reached within this progression. Use “Phase” when describing the period and “Stadium” when the current stage of development is the focus.

Die Pflanze ist inzwischen in einem fortgeschrittenen Stadium ihrer Entwicklung.
The plant is now at an advanced stage of its development.
In der Blütephase braucht die Pflanze besonders viel Wasser.
During the flowering phase, the plant needs particularly large amounts of water.