freilassen
to release
To release a person or animal from captivity, detention, or an enclosure into freedom.
Der Zoo hat den genesenen Vogel wieder in der Wildnis freigelassen.
The zoo released the recovered bird back into the wild.

to release
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to release
To release a person or animal from captivity, detention, or an enclosure into freedom.
Der Zoo hat den genesenen Vogel wieder in der Wildnis freigelassen.
The zoo released the recovered bird back into the wild.

'Freilassen' (to release) is a separable verb. The prefix 'frei-' comes at the end in a main clause. In subordinate clauses and in the infinitive with 'zu', the verb stays together.
Der Fischer lässt den kleinen Fisch wieder frei.
The fisherman releases the small fish again.
The verb is made up of 'frei' (free) and 'lassen' (to let/leave). The meaning is literal: to let someone or something become free, or to release them into freedom.
Nach dem Ende des Experiments wurden die Versuchstiere freigelassen.
After the end of the experiment, the test animals were released.
In this meaning, 'freilassen' (to release) is often combined with living beings. Typical phrases are 'Tiere in die Wildnis freilassen' (to release animals into the wild), 'Gefangene freilassen' (to release prisoners), or 'Geiseln freilassen' (to release hostages).
Die Organisation kämpft dafür, die politischen Gefangenen freizulassen.
The organization is fighting to have the political prisoners released.
This meaning is used almost exclusively in scientific or technical contexts. The object is then usually something abstract such as 'Energie' (energy), 'Wärme' (heat), or chemical 'Stoffe' (substances).
Wenn das Material erhitzt wird, lässt es Gase frei.
When the material is heated, it releases gases.
to catch
'fangen' and 'freilassen' form a clear contrast here. With 'fangen', you put an animal or a person into a situation where it or they can no longer get away. With 'freilassen', you end this captivity again and give freedom back. This contrast fits especially with animals, prisoners, or people being held. In short: when catching, you take freedom away; when releasing, you give it back.

Nach der Behandlung lässt die Tierärztin den Vogel wieder frei.
After the treatment, the vet releases the bird again.
Der Junge fängt den verletzten Vogel vorsichtig mit einem Tuch.
The boy carefully catches the injured bird with a cloth.