bedeutend
significant
Very important, influential, or of great consequence.
Die Erfindung des Buchdrucks war ein bedeutendes Ereignis in der Geschichte.
The invention of printing was a significant event in history.

significant / considerable
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significant
Very important, influential, or of great consequence.
Die Erfindung des Buchdrucks war ein bedeutendes Ereignis in der Geschichte.
The invention of printing was a significant event in history.

'Bedeutend' (significant/considerable) is the present participle of the verb 'bedeuten' (to mean, to signify). So it describes something that has or gives meaning.
Das ist ein bedeutender Fortschritt für die Medizin.
That is a significant advance for medicine.
'Bedeutend' can stand before a noun as an adjective or be used as an adverb to intensify another adjective, often in the comparative. As an adverb, it means 'considerably' or 'significantly'.
Sie ist eine bedeutende Persönlichkeit in der Politik.
She is a significant figure in politics.
If 'bedeutend' refers to a person, an event, or an idea, it means 'important' or 'significant'. If it comes before a comparative, e.g. 'größer' (larger), or a word for quantity, e.g. 'Teil' (part), it means 'considerable' or 'considerably'.
Marie Curie war eine bedeutende Wissenschaftlerin.
Marie Curie was a significant scientist.
In the meaning 'important', 'bedeutend' is often combined with people, events, or abstract concepts: 'eine bedeutende Rolle' (a significant role), 'ein bedeutender Schritt' (a significant step), 'ein bedeutender Künstler' (a significant artist).
Die Unterzeichnung des Vertrags war ein bedeutender Schritt zum Frieden.
The signing of the treaty was a significant step toward peace.
big / great
'groß' and 'bedeutend' can both say that something is important or strong in extent. 'groß' is more general and often also more emotional or everyday. For example, you say 'ein großer Erfolg' (a great success) or 'eine große Hilfe' (a great help). 'bedeutend' sounds more factual and emphasizes that something is important, influential, or substantial. It fits especially well in reports, news, or more serious conversations. For simple everyday language, 'groß' is often more natural; for importance with an effect, 'bedeutend' is more precise.

Im Gespräch nennt Lukas einen bedeutenden Grund für die Entscheidung.
In the conversation, Lukas mentions a significant reason for the decision.
Die Unterstützung meiner Freunde war eine große Hilfe.
The support from my friends was a great help.