a.

1. 无效的；无价值的
2. 【数】零的

n.

1. 【数】零

vt.

1. 使无效

adj.

1. (usu. in phr. null and void) having no legal force; invalid.
2. having or associated with the value zero.
   ▸Mathematics  (of a set or matrix) having no elements, or only zeros as elements.
3. having no positive substance.

n.

1. a dummy letter in a cipher.
2. Electronics  a condition in which no signal is generated; something generating no signal.

v.