noun

das null, –

consider

Etwas auf eine bestimmte Art sehen oder meinen, dass es so ist.

Meaning (in English):
Erachten means to consider, to deem, to regard something as something. It is a rather formal/official verb and is often used in administrative, legal, or written contexts.

Common constructions

etw. als … erachten = to consider/deem something as …
- Ich erachte das als sinnvoll. = “I consider that sensible.”
etw. für … erachten = to deem something (to be) … / to consider something …
- Man erachtet es für notwendig. = “It is deemed necessary.”

Parts of the word

er-: a common German prefix often used to form verbs (frequently with a sense of “bringing about” a result).
achten: “to pay attention to; to respect; to value” (depending on context)

So erachten developed the sense of “to judge/consider something (as…)”.

German synonyms

Depending on context:

halten für (less formal)
ansehen als
betrachten als
meinen (more subjective)
finden (more colloquial)
für … halten
als … einstufen (more technical/administrative)
als … bewerten

German antonyms (where applicable)

There is no single direct antonym in all contexts, but common opposites include:

ablehnen (to reject)
nicht für … halten / nicht als … ansehen
verwerfen (formal: to dismiss/reject)
missbilligen (to disapprove)

Notes on usage

Erachten is less common in everyday spoken German and sounds formal or written.
It typically expresses a judgment or assessment, often with an implication of authority or deliberation.