The answer to this question would be: an organ
A group of a similar cell will make a tissue, but a group of tissues will be called an organ. In the organ, there is various tissue that will work together to do a function.
In the heart, the muscle tissue will contract and pump the blood. The connective tissue will make the valve so the blood will be pumped in one way direction. Every tissue will do different jobs and if you lose one of them it will decrease the organ capability.