Wiki定义：In computer science, beam search is a heuristic search algorithm that explores a graph by expanding the most promising node in a limited set. Beam search is an optimization of best-first search that reduces its memory requirements. Best-first search is a graph search which orders all partial solutions (states) according to some heuristic which attempts to predict how close a partial solution is to a complete solution (goal state). But in beam search, only a predetermined number of best partial solutions are kept as candidates.
束搜索是一种启发式搜索算法，通过从有限集中扩展最有可能的节点来遍历图。束搜索是对优先搜索（best-first search）的优化，降低内存需求。优先搜索是一种图搜索，它根据一些启发式的规则来对所有局部解进行排序，试图找到与全局解最近的局部解。但是在束搜索中，仅保留预定义数目的最优部分解。
最近好多paper中都用到了beam search。
（1）Learning when Training Data are Costly: The Effect of Class Distribution on Tree Induction, JAIR 2003
确定训练数据中类别分布的算法中用到。
