(Latin: ad, to; tendere, to stretch)
Earnest direction of the senses and mind to some occupation or duty, as prayer and administration of the sacraments.
In vocal prayer it implies mindfulness of the words or purpose of the prayer; in mental, attention to the matter of meditation and to the spiritual affections aroused.
In administering the sacraments sufficient attention is required to avoid any distraction that might occasion a mistake, and at the time of the Consecration at Mass avoidance of wilful distraction.
The obligation to recite the Divine Office is satisfied by avoiding any action that is inconsistent, but to recite it properly some internal attention to the words is required.
So also external attention suffices for fulfilling the obligation of assisting at Mass, but internal attention is required to assist at it with due devotion.
New Catholic Dictionary