speisen
to dine
In elevated language, used for eating a meal, often in a formal or festive setting.
Wir werden heute Abend im Sternerestaurant speisen.
We will dine at the Michelin-starred restaurant this evening.

to dine / supply
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to dine
In elevated language, used for eating a meal, often in a formal or festive setting.
Wir werden heute Abend im Sternerestaurant speisen.
We will dine at the Michelin-starred restaurant this evening.

The verb 'speisen' (to dine/feed) is closely related to the noun 'die Speise' (the food, the dish). This connection helps you understand the basic meaning of eating.
Als Vorspeise gab es einen Salat.
There was a salad as the appetizer.
You can recognize the meaning from the context: If it is about people and food, 'speisen' (to eat/dine) is meant. In technology and energy contexts, 'speisen' (to supply/feed) is meant. The reflexive form 'sich speisen aus' (to stem from) is used for abstract sources.
Die Gäste speisten im großen Saal des Schlosses.
The guests dined in the large hall of the castle.
In the meaning 'eine Mahlzeit einnehmen' (to have a meal), 'speisen' (to dine) is used mainly in elevated or formal language. It sounds more elegant and festive.
Die königliche Familie speist jeden Abend um Punkt acht Uhr.
The royal family dines every evening at exactly eight o'clock.
When it is about supplying energy or data, 'speisen' (to feed/supply) is a technical term. The subject is often a device or a system.
Das Kraftwerk speist Strom ins Netz.
The power plant feeds electricity into the grid.
This meaning is always used in the reflexive construction 'sich speisen aus' (to stem from) + dative. It describes the source or origin of something abstract, such as feelings or ideas.
Sein Hass speist sich aus einem tiefen Gefühl der Ungerechtigkeit.
His hatred stems from a deep feeling of injustice.
to eat
'essen' and 'speisen' can both mean eating a meal. 'essen' is the normal and neutral word for everyday life, family, work, and restaurants. 'speisen' sounds elevated, ceremonial, or somewhat old-fashioned. It is used more for a festive meal, in a very elegant restaurant, or in formal texts. In normal situations, 'speisen' often sounds too strong or ironic.

Nach der Hochzeit speisten die Gäste in einem eleganten Saal.
After the wedding, the guests dined in an elegant hall.
Vor dem Termin isst meine Nachbarin noch schnell ein Brot.
Before the appointment, my neighbor quickly eats a slice of bread.