An attaché was an individual assigned to the staff of an institution or a higher placed individual. A strategic attaché was a specialized type of this.
As a junior officer, Lisa Cusak was assigned to the Federation embassy on Andor as an attaché. (DS9: "The Sound of Her Voice")
After Starfleet took control of Deep Space 9 in 2369, Major Kira Nerys was assigned to the station as attaché and first officer by the Bajoran Provisional Government. (DS9: "Emissary")
In later episodes, Kira was always described as being the station's liaison officer.
In 2370, Calvin Hudson was Starfleet's attaché to the Federation colonies in the new Demilitarized Zone. (DS9: "The Maquis, Part I")
In 2375, Lieutenant Curtis was the attaché to Vice Admiral Matthew Dougherty. (Star Trek: Insurrection)