zenith
the point in the sky directly above an observer on Earth

Z line
the line formed by the attachment of actin filaments between two sarcomeres of a muscle fiber in striated muscle cells

zone of aeration
the zone between the ground surface and the water table

zone of saturation
the zone of rock or soil below the water table; the zone in which all void spaces are filled with water

zooplankton
microscopic animal organisms that drift in bodies of water worldwide; represent a basic level of feeding relationships

zoospore
a flagellated, motile spore produced by algae and some protozoans

zygosporangium
in members of the phylum Zygomycota, a sexual structure that is formed by the fusion of two gametangia and that contains one or more zygotes that resulted from the fusion of gametes produced by the gametangia

zygospore
in some algae, a thick-walled protective structure that contains a zygote that resulted from the fusion of two gametes

zygote
the cell that results from the fusion of gametes; a fertilized egg