der Hinterkopf
back of the head
The rear, lower part of the head.
Er lehnte sich zurück und stützte den Hinterkopf mit den Händen.
He leaned back and supported the back of his head with his hands.

back of the head
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back of the head
The rear, lower part of the head.
Er lehnte sich zurück und stützte den Hinterkopf mit den Händen.
He leaned back and supported the back of his head with his hands.

'Hinterkopf' (back of the head) is a compound noun made from the preposition 'hinter' (behind) and the noun 'Kopf' (head). So it very logically describes the back part of the head.
Das Kind hat eine kleine Beule am Hinterkopf.
The child has a small bump on the back of their head.
The literal meaning refers to the body part and is often connected with physical actions. You can recognize the figurative meaning by the verbs 'haben' (to have) or 'behalten' (to keep) in the fixed expression 'etwas im Hinterkopf haben/behalten' (to have/keep something in mind).
Er fiel unglücklich auf den Hinterkopf.
He fell awkwardly on the back of his head.
In the literal meaning, 'Hinterkopf' (back of the head) is often used with the prepositions 'am' (position) or 'auf' (movement/impact).
Er kratzte sich verlegen am Hinterkopf.
He scratched the back of his head in embarrassment.
The expression 'etwas im Hinterkopf behalten' (to keep something in mind) means to remember something without thinking about it constantly. The 'etwas' (something) is an accusative object here.
Behalte bitte im Hinterkopf, dass wir morgen früh losfahren müssen.
Please keep in mind that we have to leave early tomorrow morning.
thought
'der Gedanke' and the phrase 'etwas im Hinterkopf haben' (to keep something in mind) both have to do with something that is in the mind. A thought is a concrete idea, consideration, or intention that you can think about. When you have something 'im Hinterkopf', you are not actively thinking about it all the time, but you keep it as important information for later. The words can be confusing because 'Hinterkopf' is used figuratively here. It is not about the body part, but about something you do not want to forget.

Den Termin am Montag habe ich noch im Hinterkopf.
I still have the appointment on Monday in mind.
Der Gedanke an einen Umzug macht Sofia nervös.
The thought of moving makes Sofia nervous.