Another excerpt by Billy Collins from his introduction to Haiku in English, edited by Jim Kacian, ©2013:
Haiku is both easy and impossible to define. One can merely use dictionary language to say that a haiku is a short poem, usually in three lines, that uses natural imagery to evoke a feeling or mood. But such flat definitions fall well short of accounting for haiku’s mysterious power to cause in the reader’s consciousness a sudden shift, literally a new way of seeing. Part of this ability lies in the form’s brevity, which leaves no time to explain an experience; instead, the haiku conveys an experience directly without commentary and with an immediacy not possible in longer poems.