Surprise

In combat, the element of surprise triggers a surprise combat round where the attacker gains the opportunity to take action prior to regular combat rounds starting.
  If the attacker is unseen and unexpected, the attacker gains an advantage on their attack and gains the first attack before combat begins.
  If the attacker is unexpected, the attacker gains the first attack before combat begins without any advantage.
  In a situation where some characters are surprised but some are not, those who are not surprised (and are expecting the action) can participate in the pre-combat surprise round.