Dictionary.net

• the highest level of alert when an attack by the enemy seems immanent (or more generally a state of alert resulting from immanent danger)

2006 - WordNet 3.0
By Princeton University

• the highest level of alert when an attack by the enemy seems immanent (or more generally a state of alert resulting from immanent danger)

2011 - English Dictionary Database
By DataStellar Co., Ltd