A quick introduction into baculoviruses and baculovirus expression vectors, brought to you by OET.
Short answer: An insect virus which over the years has been used for recombinant protein expression by some very clever people.
Long answer: A baculovirus is a member of a family of rod shaped, double stranded DNA viruses. The predominately infect insect larvae of the order of Lepidoptera (butterflies and moths).
A baculovirus expression vector is a recombinant baculovirus that has been modified to contain a new gene of interest, which can then be expressed in insect cells under the control of a baculovirus gene promoter.
The most commonly used baculovirus is Autographa californica nucleopolyhedrovirus, what a mouthful! AcMNPV for short. AcMNPV has a circular, double stranded, super-coiled DNA genome (133894 bp; Accession: NC_001623) all packaged in a rod shaped nucleocapsid. The properties of this nucleocapsid allow it to extend lengthways which can accommodate large insertions of foreign DNA, very useful when expressing larger proteins.