admission of validity made to an authority or witnesses
The child's biological father was entitled to challenge the acknowledgment of paternity executed (=signed) by the mother and her former lover under the statute.
response confirming that an act has been performed
She failed to file either an acknowledgment of service or proof of service of the summons.
response confirming that something has been received
She signed the employee acknowledgement form, acknowledging receipt of the "Fair Treatment Process" brochure.