What is defined as an untruthful spoken attack on someone's reputation?

The term that specifically refers to an untruthful spoken attack on someone's reputation is "slander." Slander falls under the broader legal concepts of defamation, which encompasses both spoken and written statements. In the context of defamation, slander is used to describe harmful statements that are articulated verbally and can lead to reputational damage.

While "defamation" is the overarching category for both slander and libel (the latter being written defamation), the key aspect here is the spoken nature of the attack, which is why slander is the most accurate choice. Other options like calumny also pertain to false statements damaging someone's reputation but are not as specific in identifying the form of communication as slander does.