Sapphism (adj. sapphic) is a retool term that refers to women who have romantic and/or sexual attraction to other women. What separates sapphism from lesbianism is inclusivity. Specifically, sapphic is an adjective that includes not only lesbians, but bisexual, pansexual, and homo/bi/pan-romantic asexual women, as well some non-binary transgender individuals.
The term comes from famous Greek poet Sappho who kept a cadre of women she admired and loved with her on the island of Lesbos. Using the term sapphic in communication is useful to reduce presumptive errors about another person, both in sexual orientation and gender identity.
Usage of the term has gained popularity in progressive and LGBTQ spaces, especially as a less gender-restricted term. Many transgender people prefer to use the terminology to describe themselves if they identify as wholly or partially female and/or non-binary. In this way, they can escape a lot of the excessively-gendered tropes attached to the term, such as Butch Lesbian and Lipstick Lesbian.
In general, the term is also used to avoid attachment to scores of previous organizations and movements that have flavored and defined what it truly means to be "lesbian" or a woman who likes women.