Hate speech is a term with varied meaning and has no single, consistent definition. Cambridge Dictionary defines hate speech as "public speech that expresses hate or encourages violence towards a person or group based on something such as race, religion, sex, or sexual orientation". The Encyclopedia of the American Constitution states that hate speech is "usually thought to include communications of animosity or disparagement of an individual or a group on account of a group characteristic such as race, color, national origin, sex, disability, religion, or sexual orientation". Hate speech can include incitement based on social class or political beliefs. There is no single definition of what constitutes "hate" or "disparagement". Legal definitions of hate speech vary from country to country.

There has been much debate over freedom of speech, hate speech, and hate speech legislation. The laws of some countries describe hate speech as speech, gestures, conduct, writing, or displays that incite violence or prejudicial actions against a group or individuals on the basis of their membership in the group, or that disparage or intimidate a group or individuals on the basis of their membership in the group. The law may identify protected groups based on certain characteristics. In some countries, a victim of hate speech may seek redress under civil law, criminal law, or both. In the United States, what is usually labelled "hate speech" is constitutionally protected.

Hate speech is generally accepted to be one of the prerequisites for mass atrocities such as genocide. Incitement to genocide is an extreme form of hate speech, and has been prosecuted in international courts such as the International Criminal Tribunal for Rwanda.

Early hate speech laws were enacted in the 1820s in France and 1851 in Prussia.

Starting in the 1940s and 50s, various American civil rights groups responded to the atrocities of World War II by advocating for restrictions on hateful speech targeting groups on the basis of race and religion. These organizations used group libel as a legal framework for describing hate speech and addressing its harm. In his discussion of the history of criminal libel, scholar Jeremy Waldron states that these laws helped "vindicate public order, not just by preempting violence, but by upholding against attack a shared sense of the basic elements of each person's status, dignity, and reputation as a citizen or member of society in good standing". A key legal victory for this view came in 1952 when group libel law was affirmed by the United States Supreme Court in Beauharnais v. Illinois. However, the group libel approach lost ground due to a rise in support for individual rights within civil rights movements during the 60s. Critiques of group defamation laws are not limited to defenders of individual rights. Some legal theorists, such as critical race theorist Richard Delgado, support legal limits on hate speech, but claim that defamation is too narrow a category to fully counter hate speech. Ultimately, Delgado advocates a legal strategy that would establish a specific section of tort law for responding to racist insults, citing the difficulty of receiving redress under the existing legal system.

The rise of the internet and social media has presented a new medium through which hate speech can spread. Hate speech on the internet can be traced all the way back to its initial years, with a 1983 bulletin board system created by neo-Nazi George Dietz considered the first instance of hate speech online. As the internet evolved over time hate speech continued to spread and create its footprint; the first hate speech website Stormfront was published in 1996, and hate speech has become one of the central challenges for social media platforms.

The structure and nature of the internet contribute to both the creation and persistence of hate speech online. The widespread use and access to the internet gives hate mongers an easy way to spread their message to wide audiences with little cost and effort. According to the International Telecommunication Union, approximately 66% of the world population has access to the internet. Additionally, the pseudo-anonymous nature of the internet imboldens many to make statements constituting hate speech that they otherwise wouldn't for fear of social or real life repercussions. While some governments and companies attempt to combat this type of behavior by leveraging real name systems, difficulties in verifying identities online, public opposition to such policies, and sites that don't enforce these policies leave large spaces for this behavior to persist.

Because the internet crosses national borders, comprehensive government regulations on online hate speech can be difficult to implement and enforce. Governments who want to regulate hate speech contend with issues around lack of jurisdiction and conflicting viewpoints from other countries. In an early example of this, the case of Yahoo! Inc. v. La Ligue Contre Le Racisme et l'Antisemitisme had a French court hold Yahoo! liable for allowing Nazi memorabilia auctions to be visible to the public. Yahoo! refused to comply with the ruling and ultimately won relief in a U.S. court which found that the ruling was unenforceable in the U.S. Disagreements like these make national level regulations difficult, and while there are some international efforts and laws that attempt to regulate hate speech and its online presence, as with most international agreements the implementation and interpretation of these treaties varies by country.