This is the 5660th most frequent Russian word.
президентского
"Presidential" (possessive form related to "president").
The word 'президентского' functions as an adjective modifying the noun 'указа' ('decree'), describing the decree as related to the president.
Президентского указа следует придерживаться.
One should adhere to the presidential decree.
Here, 'президентского' is an adjective describing the noun 'самолета' ('airplane'), indicating the airplane associated with the president.
Расписание президентского самолета было изменено.
The schedule of the presidential airplane was changed.
In this sentence, 'президентского' modifies the noun 'срока' ('term'), specifying the term related to the president.