This is a allusion/euphemistic for “lesbianism.” Though, weirdly, the reference is circular.
Sappho was a female Greek poet in the 6th century BC. She was born and lived at least part of her life on the Greek island of Lesbos.
She may or may not have been homosexual, though most scholars believe she was. She wrote about love between women, but there is some disagreement whether it can be interpreted as platonic or erotic love.
For example, Wikipedia explains in an article about Sappho’s most famous work Ode to Aphrodite:
The speaker is identified in the poem as Sappho, in one of only four surviving works where Sappho names herself. The sex of Sappho’s beloved is established from only a single word, the feminine εθελοισα in line 24. […]
As near as I can tell, here is stanza from the poem (I find the female reference in line 21 of the entire poem, not line 24 as stated above)
‘See, if now she flies, she soon must follow;
Yes, if spurning gifts, she soon must offer;
Yes, if loving not, she soon must love thee,
However, regardless of whether or not Sappho was actually gay, she has become a symbol of female homosexuality.
The demonym for the island of Lesbos is “Lesbian” (demonym: someone from America is an “American”; someone from Lesbos is a “Lesbian”). Thus, Sappho – as the most famous person from Lesbos and a symbol of female homosexuality – is unwittingly responsible for “lesbian” having the modern meaning that it does.
Therefore, the definition of Sapphism is circular: Sappho implies “lesbian” but “lesbian” only carries the meaning it does because of Sappho.