verbindlich
binding
Legally or morally binding and obligatory. A binding commitment must be honored.
Erst mit Ihrer Unterschrift wird der Vertrag für beide Seiten verbindlich.
Only with your signature does the contract become binding for both sides.

binding / courteous
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binding
Legally or morally binding and obligatory. A binding commitment must be honored.
Erst mit Ihrer Unterschrift wird der Vertrag für beide Seiten verbindlich.
Only with your signature does the contract become binding for both sides.

The adjective 'verbindlich' (binding/courteous) is closely related to the verb 'verbinden' (to connect). It describes something that creates a connection—either a legal obligation or a friendly social relationship.
Eine mündliche Zusage ist oft nicht rechtlich verbindlich.
An oral promise is often not legally binding.
You can recognize the meaning from the context. If it is about contracts, rules, or commitments, the legal meaning is intended. If it is about dealing with people, tone of voice, or a smile, the polite manner is meant.
Bitte geben Sie mir eine verbindliche Antwort bis morgen.
Please give me a binding answer by tomorrow.
In the legal meaning, 'verbindlich' (binding) is often combined with nouns from business or administration: eine verbindliche Zusage (a binding commitment), ein verbindliches Angebot (a binding offer), eine verbindliche Auskunft (binding information), or eine verbindliche Regelung (a binding regulation).
Ist dieses Angebot für den Verkäufer verbindlich?
Is this offer binding for the seller?
When it is about politeness, 'verbindlich' (courteous) often describes the manner of communication: ein verbindlicher Ton (a courteous tone), ein verbindliches Lächeln (a courteous smile), or eine verbindliche Art (a courteous manner).
Sie begrüßte die Gäste mit einem verbindlichen Lächeln.
She greeted the guests with a courteous smile.
firm / fixed
'fest' and 'verbindlich' can both mean that something is not just loose or uncertain. With appointments/dates, commitments, or rules, there is some overlap. 'fest' emphasizes that something is clearly set and is not meant to change anymore. 'verbindlich' emphasizes more strongly that you must stick to it, often legally or morally. Learners may confuse the words because both fit with agreements. An appointment/date can be 'fest'; a commitment or rule is 'verbindlich' if it truly obligates you.

Die Anmeldung zum Kurs ist verbindlich und kann nicht einfach storniert werden.
Registration for the course is binding and cannot simply be canceled.
Frau Fischer hat den Termin am Telefon fest zugesagt.
Ms. Fischer firmly confirmed the appointment on the phone.