There are two very popular software "Turnitin" and "iThenticate" these are being used by most of the universities and journals. Moreover, we can also check the similarity and plagiarism by "Grammarly" and few other similar websites.
Although, the Turnitin and iThenticate both are the plagiarism checker software and also use to verify the originality of work. However, these give the results in the form of a similarity index. So, in other words, these software describes us that how much of our document is similar to other already published work and then we can rephrase our document to minimize the similarity index. While plagiarism in the citing of other person work or idea as our own without giving credit to the actual authors. All researchers must avoid the plagiarism because in scientific work it is not acceptable. It is not morally correct but sometimes it also has legal consequences as well.
You are right. Turnintin and iThenticate both show the similarity index not the plagiarism. All these software show similarity index not the plagiarism but they are called plagiarism checker software.
Well as per my understanding similarity index is found by iThenticate and Turnitin but plagiarism you have to decide after you see the reports. In other words "Computers have no brain you have to use your own"
I have been using Turnitin since 2007, I also used iThenticate for few years. A sensible writer can see what is similarity and what is plagiarism? only once sincere with the writing profession.