mehrheitlich
by majority
Supported by the majority of a group or applying to the largest part of something; for the most part.
Der Vorschlag wurde vom Ausschuss mehrheitlich angenommen.
The proposal was accepted by majority vote by the committee.

by majority / predominantly
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by majority
Supported by the majority of a group or applying to the largest part of something; for the most part.
Der Vorschlag wurde vom Ausschuss mehrheitlich angenommen.
The proposal was accepted by majority vote by the committee.

The word is derived from 'die Mehrheit' (the majority). The ending '-lich' turns the noun into an adjective or adverb with the meaning 'like the majority' or 'for the most part'.
Die Mehrheit der Bürger hat für den Kandidaten gestimmt.
The majority of citizens voted for the candidate.
'Mehrheitlich' (by majority / predominantly) can be used as an adverb without an ending or as a declined adjective before a noun. As an adverb, it describes the manner of an action.
Die Aktionäre stimmten mehrheitlich zu.
The shareholders agreed by majority vote.
In the meaning 'von der Mehrheit' (by the majority), the word is often combined with verbs that express a decision or opinion: 'annehmen' (to accept), 'ablehnen' (to reject), 'beschließen' (to decide), or 'zustimmen' (to agree).
Der Antrag wurde vom Parlament mehrheitlich abgelehnt.
The motion was rejected by majority vote by parliament.
partly
'teilweise' and 'mehrheitlich' both have to do with a part of something, but they indicate different sizes. 'teilweise' only says that something does not apply completely; the part can also be small. 'mehrheitlich' means that the majority or the largest part is affected. It can refer to people, decisions, or the composition of something. The words can be confused because both do not mean 'all'. The important difference is: 'teilweise' emphasizes the limitation, while 'mehrheitlich' emphasizes the predominant part.

Die Klasse stimmt mehrheitlich für den Ausflug am Freitag.
The class votes by majority for the trip on Friday.
Du hast den Text teilweise verstanden, aber die letzten Abschnitte sind noch schwer.
You partly understood the text, but the last sections are still difficult.