das Gegenteil
opposite
Something that is completely opposed to another thing in its qualities or meaning.
Hass ist das Gegenteil von Liebe.
Hate is the opposite of love.

opposite
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opposite
Something that is completely opposed to another thing in its qualities or meaning.
Hass ist das Gegenteil von Liebe.
Hate is the opposite of love.

The word is made up of 'gegen' (against) and 'Teil' (part). So it literally describes the 'part' that stands opposed to something.
Das Gegenteil von „heiß“ ist „kalt“.
The opposite of 'heiß' (hot) is 'kalt' (cold).
When 'Gegenteil' (opposite) has the meaning 'opposite thing', it is almost always used with the preposition 'von' and the dative to name the reference point.
Sein Verhalten war das genaue Gegenteil von dem, was er versprochen hatte.
His behavior was the exact opposite of what he had promised.
The fixed expression 'im Gegenteil' (on the contrary) often appears at the beginning of a sentence to correct a previous statement. In this position, it introduces a main clause and requires inversion (the verb comes directly after it in position 2).
Er ist nicht faul. Im Gegenteil, er arbeitet mehr als alle anderen.
He is not lazy. On the contrary, he works more than everyone else.
against it / however
'dagegen' and 'das Gegenteil' both have to do with opposition, but they are used differently. 'dagegen' is an adverb and shows objection or contrast to something mentioned before: you are against something, or something is different in contrast. 'das Gegenteil' is a noun and means the thing that is completely opposite to another. In the fixed expression 'im Gegenteil' (on the contrary), you reject a statement and emphasize that the reverse is true. They can be confused because both express an opposite direction. Grammatically, this rule helps: 'dagegen' works like a small linking word in the sentence, while 'das Gegenteil' needs an article and functions like a noun.

Kalt ist das Gegenteil von warm.
Cold is the opposite of warm.
Du findest den Plan gut, aber ich bin dagegen.
You think the plan is good, but I am against it.