der Fachmann
expert
A person who has special knowledge and great skill in a particular field; an expert.
Für die Reparatur der Heizung sollten wir einen Fachmann rufen.
We should call an expert to repair the heating system.

expert
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expert
A person who has special knowledge and great skill in a particular field; an expert.
Für die Reparatur der Heizung sollten wir einen Fachmann rufen.
We should call an expert to repair the heating system.

The word is made up of 'Fach' (field/subject area) and 'Mann' (man). So it literally describes a man who knows a lot about a particular field.
Bei Computerproblemen frage ich immer meinen Kollegen, er ist da der Fachmann.
When I have computer problems, I always ask my colleague; he's the expert there.
To name the area of expertise, German usually uses the preposition 'für' (for) with the accusative or the phrase 'auf dem Gebiet' (in the field) with the genitive.
Wir suchen einen Fachmann für Steuerrecht.
We are looking for an expert in tax law.
In a professional context, an expert is often 'gerufen' (called), 'geholt' (brought in), 'beauftragt' (hired/commissioned), or 'konsultiert' (consulted) when their help is needed.
Für die Renovierung haben wir einen Fachmann beauftragt.
We hired an expert for the renovation.
In the figurative sense, you can often recognize the meaning from additions like 'ein echter Fachmann' (a real expert) or 'ein wahrer Fachmann' (a true expert). The context is usually a hobby or everyday knowledge, not a profession.
Was das Grillen angeht, ist mein Vater ein echter Fachmann.
When it comes to grilling, my father is a real expert.
expert
'der Experte' and 'der Fachmann' are very close in meaning. Both refer to a person who knows a field well and is highly skilled. 'Experte' often sounds somewhat more general or official, for example in the media, consulting, or research. 'Fachmann' often sounds more practical and closer to a profession or trade, but it can also be used figuratively for someone with a lot of experience. The words are confusable because they are often interchangeable. When it is about specialized knowledge, both fit; for practical work, 'Fachmann' often sounds especially natural.

Zum Einbau der neuen Heizung kommt heute ein Fachmann.
A specialist is coming today to install the new heating system.
Frag lieber einen Experten, bevor du die alten Daten löschst.
It's better to ask an expert before you delete the old data.