This is the 594th most frequent Hebrew word.
לאחד
"To unite" or "to merge."
Here 'לאחד' is used as an infinitive verb meaning 'to unify' or 'to bring together'.
הם ניסו לאחד את הדעות המנוגדות לקראת ההצבעה.
They tried to unify the opposing opinions before the vote.
Here 'לאחד' introduces 'אחד ההרצאות' (one of the lectures), functioning as a preposition indicating 'after one of'.
לאחד ההרצאות הפרופסור ענה על שאלות.
After one of the lectures, the professor answered questions.
In this sentence, 'לאחד' introduces 'אחד הארוחות' (one of the meals), functioning as a preposition indicating 'for one of'.