A science fair was an event at which participants showcase scientific research that they performed in a chosen area of study. The displayed research would then be judged with prizes given to those deemed winners. Such events were normally intended for children to display work that they performed as part of a school assignment.
In 2370, the daughter of Ensign Kelly worked with a Bajoran girl on a prize-winning science fair project. Benjamin Sisko cited this as a tie Deep Space 9 personnel had with the Bajorans which was threatened by the evacuation of Starfleet personnel due to the activities of the Alliance for Global Unity. (DS9: "The Siege")