The Dark People call April a Wave, someone who's actions affect the entire universe for all time. Benrime Salmin also notes this, telling her that most people's fates pull them along, whereas April makes her own. When April asks her how she knows this, Salmin replies that it is because she is a Seer.
The Dark Person also tells April to "take [her] Wave into the future", so it appears that the term Wave can designate either a person or their effects.
The concept of a person affecting the universe around them is not uncommon in fantasy literature and games. In for instance Robert Jordan's Wheel of Time we find the concept of Ta'veren, which is reminiscent of being a Wave.
April being a Wave is slightly contrasting with Cortez' statement that she is no chosen one, but the two do not fully contradict each other, in that it is possible to be 'special' without being 'unique' and that one can choose one's own actions, as April does in accepting the seemingly impossible and stepping through the shift into Arcadia.