lebenslang
lifelong
Something that applies or lasts for the entire duration of a person's life.
Er erhielt eine lebenslange Garantie auf das Produkt.
He received a lifelong warranty on the product.

lifelong / life
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lifelong
Something that applies or lasts for the entire duration of a person's life.
Er erhielt eine lebenslange Garantie auf das Produkt.
He received a lifelong warranty on the product.

The word is made up of 'Leben' (life), the linking element '-s-', and 'lang' (long). So it literally means 'for a life long' and is therefore easy to understand.
Sie verband eine lebenslange Freundschaft.
They were connected by a lifelong friendship.
'Lebenslang' can stand before a noun as an adjective or function as an adverb referring to a verb. As an adverb, it is not declined.
Er bekam eine lebenslange Haftstrafe.
He received a life sentence.
In its general meaning, 'lebenslang' (lifelong) is often connected with abstract concepts, such as 'lebenslanges Lernen' (lifelong learning), a 'lebenslange Aufgabe' (lifelong task), or a 'lebenslange Freundschaft' (lifelong friendship).
Gute Gesundheit ist eine lebenslange Aufgabe.
Good health is a lifelong task.
In a legal context, the combination with 'Haft' (imprisonment) or 'Freiheitsstrafe' (custodial sentence/prison sentence) is most common. The phrase 'zu lebenslang verurteilt' (sentenced to life) is also often used.
Der Angeklagte wurde zu lebenslang verurteilt.
The defendant was sentenced to life.
permanent / lasting
'dauerhaft' and 'lebenslang' can both mean that something applies for a very long time. 'dauerhaft' is more general: it can refer to things, states, or rules that remain for a long time or forever. 'lebenslang' is more specific and refers to a person's entire lifetime. In legal texts, 'lebenslang' can also describe a serious prison sentence. The words are easy to confuse if you only mean 'forever'. If it is really about a person's whole life, 'lebenslang' is more precise. In other cases, 'dauerhaft' is more natural.

Nach dem Unfall braucht sie lebenslang medizinische Hilfe.
After the accident, she will need medical help for life.
Die neue Regel gilt dauerhaft für alle Mitarbeiter.
The new rule applies permanently to all employees.