Well, there is a good definition in a Merriam-Webster dictionary (I, myself, find it useful. You can argue, of course). Still, it says that defamation is
- the act of communicating false statements about a person that injure the reputation of that person, or
- the act of defaming another.
To clarify, I’d say, that defamation is the communication of a false statement that harms the reputation of a person, business, product, group, government, religion, or nation.
Under common law, defamation means that claim is false and have been made by someone other than the person defamed. Some common law jurisdictions also differ spoken defamation, aka slander, and defamation in media like printed words or images, aka libel.
There are also false light laws, which are supposed to warn against statements which are not actually wrong, but in some case misleading.
In some civil law jurisdictions, defamation is treated as a crime, not a public wrong.
I’ve also found information, that The United Nations Human Rights Committee ruled in 2012 that the libel law of one country, the Philippines, was inconsistent with Article 19 of the International Covenant on Civil and Political Rights, as well as urging that "State parties [to the Covenant] should consider the decriminalization of libel."
Also, I’d add that a person who defames another may be called a "defamer," "libeler," "slanderer," or rarely - "femicide." This is just additional to the question, not a must, of course :)