Terrorism is violence the purpose of which is to strike fear in the populace in order to generate pressure on its government to advance the political objectives of the terrorist. Though to confirm the Boston bombing is an act of terrorism we have to wait for the facts, which take time to develop and publish, it initially appears to meet the requirements of the definition.
As other comments have pointed out, terrorism is not strictly limited to any one nationality or political movement. Some cultures are far more supportive of it than others, though, and so it is heavily concentrated in them. It being not only unwise but also foolish to divert into the trap of grossly unbalanced moral equivalencies, it is those cultures that should be the focus of attention on terrorism. That leads us first to suspect violent Islamist radicals, and rightly so.
Early reports are that the hospitalized suspect is a young Saudi male, a prime profile of an Islamist terrorist. Based on the sketchy information available to us tonight, the tentative conclusion we have to reach is, as we should assume by default, Islamism is behind this act.