I have seen it used to mean the effects on a tissue of something like the CN ion cyanide. It blocks the basic energy production and/or transport system in a living cell - in the case of cyanide, binding cyctochrome C oxidase in mitochondrial membranes which inhibits ATP production.
In other words, it's a really old term for a toxin that acts inside the cell, as Monique said. This term predates knowledge of how those little "organelles" inside a cell did their jobs. The idea is that any cell is susceptible.
Other poisons were thought to act on one particular tissue or organ system, rather than any type of cell.