asertar

A rare, formal verb meaning to assert or affirm something as true, closely related to aseverar and afirmar but far less common in actual use — most modern dictionaries mark it as a rarely used or specialized (mainly legal/philosophical) term.

• El filósofo asertaba que toda verdad es provisional. — The philosopher asserted that all truth is provisional.
• El testigo asertó los hechos ante el juez. — The witness affirmed the facts before the judge.
• No basta con asertar algo; hay que probarlo. — It's not enough to merely assert something; it has to be proven.

Reaching for asertar in everyday Spanish where afirmar or aseverar sound far more natural — asertar is rare enough that many native speakers themselves would find it unusual or overly formal.