Cycle stealing is a method of accessing RAM without interfering with the CPU. It is similar to DMA for allowing I/O controllers to read or write RAM without CPU intervention. Clever exploitation of specific CPU or bus timings could permit the CPU to run at full speed without any delay if external devices may sneak in one RAM access to memory not actively participating in the CPU's current activity and complete the operations before any possible CPU conflict. Note that cycle stealing can be done only at specific break points in an instruction cycle.
Cycle stealing has been the cause of major performance degradation on machine such as the Sinclair QL, where, for economy reasons, the video RAM was not dual access. Similar reasons were there with other system and it is not used in modern systems.