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Merriam Webster
https://www.merriam-webster.com/dictionary/vaccine
HERE IS THE ORIGINAL PRE-2020 DEFINITION:

Definition of vaccine
1: a preparation that is administered (as by injection) to stimulate the body’s immune response against a specific infectious agent or disease: such as
a: an antigenic preparation of a typically inactivated or attenuated (see ATTENUATED sense 2) pathogenic agent (such as a bacterium or virus) or one of its components or products (such as a protein or toxin)
a trivalent influenza vaccine
oral polio vaccine
Many vaccines are made from the virus itself, either weakened or killed, which will induce antibodies to bind and kill a live virus. Measles vaccines are just that, weakened (or attenuated) measles viruses.
— Ann Finkbeiner et al.
… a tetanus toxoid-containing vaccine might be recommended for wound management in a pregnant woman if [greater than or equal to] 5 years have elapsed … .
— Mark Sawyer et al.
In addition the subunit used in a vaccine must be carefully chosen, because not all components of a pathogen represent beneficial immunological targets.
— Thomas J. Matthews and Dani P. Bolognesi

HERE IS THE 2020 ADDITION:

: a preparation of genetic material (such as a strand of synthesized messenger RNA) that is used by the cells of the body to produce an antigenic substance (such as a fragment of virus spike protein)
… Moderna’s coronavirus vaccine … works by injecting a small piece of mRNA from the coronavirus that codes for the virus’ spike protein. … mRNA vaccine spurs the body to produce the spike protein internally. That, in turn, triggers an immune response.
— Susie Neilson et al.
The revolutionary messenger RNA vaccines that are now available have been over a decade in development. … Messenger RNA enters the cell cytoplasm and produces protein from the spike of the Covid-19 virus.
— Thomas F. Cozza
Viral vector vaccines, another recent type of vaccine, are similar to DNA and RNA vaccines, but the virus’s genetic information is housed in an attenuated virus (unrelated to the disease-causing virus) that helps to promote host cell fusion and entry.
— Priya Kaur
NOTE: Vaccines may contain adjuvants (such as aluminum hydroxide) designed to enhance the strength and duration of the body’s immune response.