WhatsApp end-to-end encryption
It appears to be between all platforms, as its written here:

https://whispersystems.org/blog/whatsapp-complete/

"Over the past year, we've been progressively rolling out Signal Protocol support for all WhatsApp communication across all WhatsApp clients. This includes chats, group chats, attachments, voice notes, and voice calls across Android, iPhone, Windows Phone, Nokia S40, Nokia S60, Blackberry, and BB10."