Sede vacante is an expression, used in the Canon Law of the Catholic Church, that refers to the vacancy of the Episcopal sea of a particular church. It is Latin for "the seat being vacant", the seat in question being the Cathedra of the particular church. In most cases it is used in reference to the absence of a pope during the time of a Pope's death and the election of a new Supreme Pontiff.
For the Election of a Pope
O God, eternal Shepherd,
who govern your flock with unfailing care,
grant in your boundless fatherly love
a pastor for your Church
who will please you by his holiness
and to us show watchful care.
Through our Lord Jesus Christ, your Son,
who lives and reigns with you in the unity of the Holy Spirit,