拓扑学和数学的相关领域中，积空间是指一族拓扑空间的卡积，并配备了一个称为积拓扑的自然的拓扑结构。

令I为（可能无穷的）指标集，并设X_i对于I中由i所对应的每一个拓扑空间。置X = Π X_i，也即集合X_i的卡积。对于每个I中的i，我们有一个标准投影 p_i : X → X_i。X上的积拓扑定义为所有投影p_i在该拓扑下连续的最疏拓扑（也就是开集最少的拓扑）。该乘积拓扑有时也称为吉洪诺夫拓扑。

很明显，X上的乘积拓扑可以表述为形为p_i(U)的集合生成的拓扑，其中i属于I，而U是X_i的一个开集。换句话说，集合{p_i(U)}构成X上的拓扑的子基。X的子集是开的当且仅当它是(可能无穷多的)的有限个形为p_i(U)的集合的交集的并集。p_i(U)有时称为开柱，而它们的交集称为柱集。

In topology and related areas of mathematics, a product space is the cartesian product of a family of topological spaces equipped with a natural topology called the product topology. This topology differs from another, perhaps more obvious, topology called the box topology, which can also be given to a product space and which agrees with the product topology when the product is over only finitely many spaces. However, the product topology is "correct" in that it makes the product space a categorical product of its factors, whereas the box topology is too fine; this is the sense in which the product topology is "natural".