It gets confusing sometimes.
A CLOCK cycle is one wave of oscillation at the clock input pin or from the internal clock generator
A MACHINE CYCLE or 'T' cycle is how long it takes to complete one internal operation, typically either 2 or 4 clock cycles in a PIC
An INSTRUCTION CYCLE is usually the same as machine cycles because most PIC instructions complete in one operation but it may be two machine cycles in the case of jump, call or branch instructions where a second memory fetch is needed to get the destination address.
Brian.