The Association of Teaching Artists has compiled a number of answers to this question. Their own definition: A Teaching Artist is a professional visual, performing, or literary artist who works in schools and in the community. The Teaching Artist may perform for the students and teachers, may work in long term or short-term residencies in classrooms or in a community setting, or may lead in program development through involvement in curricular planning and residencies with school partners. The Teaching Artist is an educator who integrates the creative process into the classroom and the community.
And another from field expert, Eric Booth: A teaching artist (artist educator) is a practicing professional artist with the complementary skills and sensibilities of an educator, who engages people in learning experiences in, through, and about the arts.
The wonderful regional resource, the New England Consortium of Artist-Educator Professionals (NECAP) Skills Guide is also helpful to those aiming to be - or hire - a teaching artist (artist-educator).