A mesostich is a poem or passage in which the letters in the middle of successive lines spell out a word, name, or message. It is similar to an acrostic, but the hidden pattern is built from the center of the lines rather than the first letters.
The term is uncommon, and in practice it is sometimes used loosely, so readers may need to check how a particular writer is using it.
Example
Silent clouds drift over the hill
Amber light settles at dusk
Moths circle the porch lamp
Evening turns the garden still