A tanka is a short Japanese poem with a total of 31 syllables. Traditionally, a tanka was written in one long line, but it's more common to find today's version divided into five lines.
A tanka is a slightly longer version of the more familiar haiku. Most tankas take the form of five lines divided into five, seven, five, seven, and seven syllables — if you feel hampered by the typical three brief lines of a haiku, you should try writing a tanka instead. In the 8th century AD, a tanka was simply a short poem (it means "short song" in Japanese), but the term was revived and modernized in the early 1900s.