why is the play called wit?

The play itself is a metaphor for what wit is, which as Vivian explains is the personal effort and sacrifice to gain such a deep understanding of something that it becomes simple, seemingly effortless to the one who wields it. Vivian has sacrificed her life to studying poetry, to studying Donne and his wit and by doing so she is able to see the depths of life quite simply as her life is near its end.