semiditone
Noun
Meaning
A lesser third, having its terms as 6 to 5; a hemiditone.
Source: Webster's dictionaryExamples
Also ditone and semiditone are specific for Pythagorean tuning, while tone and tritone are used generically for all tuning systems.
Interestingly, despite its name, a semiditone (3 semitones, or about 300 cents) can hardly be viewed as half of a ditone (4 semitones, or about 400 cents).