EMZETT.Unicast
In short: A transmission from exactly one sender to exactly one recipient — the most common form of network communication.
In more detail: Most everyday connections (calling up a website, retrieving email) are unicast: one clearly addressed destination, one IP address. To be distinguished from broadcast (to everyone), multicast (to a defined group) and anycast (to the nearest one of a group).
In Depth
Unicast is the normal case because it corresponds to the most intuitive communication model: one IP address as a clear destination, and a reply goes back to exactly one IP address. For this, routers only need to know via which next hop a particular target network can be reached — considerably easier to manage than multicast group memberships or broadcast domain boundaries.
The other transmission types each exist for specific special cases: broadcast is used when a sender doesn’t yet know who its counterpart is at all (e.g. with DHCP discover, before a device even has an IP address), multicast for efficient distribution to many simultaneous recipients interested in the same data stream, anycast for load distribution/resilience in globally distributed services (e.g. several DNS root servers answer under the same anycast address, and the client automatically reaches the geographically nearest one). For the overwhelming majority of everyday network communication, however, unicast remains the standard model.