einseitig
one-sided
Coming from only one side or party; not based on mutuality.
Es war eine einseitige Entscheidung, die ohne Absprache getroffen wurde.
It was a one-sided decision that was made without consultation.

one-sided
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one-sided
Coming from only one side or party; not based on mutuality.
Es war eine einseitige Entscheidung, die ohne Absprache getroffen wurde.
It was a one-sided decision that was made without consultation.

The word is clearly composed of 'ein' (one) and 'seitig' (sided, derived from 'die Seite' — the side/page). The literal meaning 'having one side' is the basis for all figurative meanings, such as 'unbalanced' or 'not mutual'.
Seine Darstellung der Ereignisse war sehr einseitig.
His account of the events was very biased.
The exact meaning of 'einseitig' (one-sided) is determined by the noun. With 'Liebe' (love) or 'Entscheidung' (decision), it means 'not mutual'. With 'Bericht' (report), it means 'biased'. With 'Ernährung' (diet) or 'Belastung' (strain), it means 'not varied'.
Eine einseitige Belastung beim Sport kann zu Verletzungen führen.
One-sided strain during sports can lead to injuries.
In the meaning 'not based on mutuality', 'einseitig' (one-sided) is often used in social or legal contexts. Typical combinations are 'einseitige Entscheidung' (one-sided decision), 'einseitige Liebe' (one-sided love), or 'einseitige Kündigung' (unilateral termination).
Der Vertrag wurde vom Unternehmen einseitig geändert.
The contract was changed unilaterally by the company.
In the physical meaning, 'einseitig' (one-sided/on one side) is often used as an adverb with verbs like 'drucken' (to print), 'beschreiben' (to write on), or 'bemalen' (to paint). It then indicates that only one physical side should be worked on.
Bitte drucken Sie das Formular nur einseitig aus.
Please print the form on one side only.
mutual
'gegenseitig' and 'einseitig' both describe who an action, feeling, or relationship comes from. 'gegenseitig' means that both sides do or feel something for each other. In this context, 'einseitig' means that only one side does or feels something. So the difference is: with 'gegenseitig', something comes from both sides; with 'einseitig', it comes from only one side.

Ihre Hilfe war einseitig, weil der andere nie etwas zurückgab.
Her help was one-sided because the other person never gave anything back.
In einer guten Freundschaft unterstützen sich die Menschen gegenseitig.
In a good friendship, people support each other.