die Aktion
action
An action, deed, or undertaking that is carried out deliberately in order to achieve a specific goal.
Die Rettungsaktion der Feuerwehr war erfolgreich.
The fire department's rescue operation was successful.

action / sale
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action
An action, deed, or undertaking that is carried out deliberately in order to achieve a specific goal.
Die Rettungsaktion der Feuerwehr war erfolgreich.
The fire department's rescue operation was successful.

The word 'Aktion' (action) belongs to the word family of 'aktiv' (active) and 'agieren' (to act). All of these words have the Latin root 'actio' (action, deed) and describe that something is being done or undertaken.
Die Aktivisten planen eine Aktion, um auf das Problem aufmerksam zu machen.
The activists are planning an action to draw attention to the problem.
You can recognize the meaning of 'Aktion' (action / sale) from the context. In a supermarket or in advertising, it means a special offer. In connection with politics or social groups, it refers to a planned measure. In combinations like 'Rettungsaktion' (rescue operation), it describes a concrete action.
Diese Woche ist Käse in Aktion.
Cheese is on sale this week.
When 'Aktion' (sale / promotion) means a special offer, the fixed phrase 'in Aktion sein' (to be on sale) or 'etwas in Aktion haben' (to have something on sale) is often used. This is typical language in supermarkets and advertising brochures.
Nächste Woche haben wir Schokolade in Aktion.
Next week we'll have chocolate on sale.
'Aktion' (action) is often used as the second part of compound nouns to describe a specific type of action. Common examples are 'Rettungsaktion' (rescue operation), 'Hilfsaktion' (aid campaign), or 'Suchaktion' (search operation).
Die Suchaktion nach dem vermissten Wanderer dauerte drei Tage.
The search operation for the missing hiker lasted three days.
company / undertaking
'das Unternehmen' and 'die Aktion' can both describe a planned action with a goal. 'Aktion' usually means something concrete, often short-term or public, for example 'eine Hilfsaktion' (a relief campaign) or 'eine Protestaktion' (a protest action). 'Unternehmen' in this meaning sounds bigger and often more difficult or risky. Learners may confuse the words because both can stand for an undertaking. When it is about a single measure or action, 'Aktion' is more natural. When the undertaking is extensive and requires a lot of effort, 'Unternehmen' fits better.

Die Schüler starten eine Aktion, um Geld für das Tierheim zu sammeln.
The students are starting a campaign to raise money for the animal shelter.
Die Rettung der alten Brücke ist ein großes Unternehmen für die ganze Stadt.
Saving the old bridge is a major undertaking for the whole city.