Simultaneous, All At Once
noun
At one moment it happened simultaneously. It was simultaneous. It was all at once.
いっせい
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own. Watch out for the shortening of 一.
生徒達は一斉に温度計に目をやりました。
The students looked at the thermometer simultaneously.