What is dhamma?

D hamma is everything which concerns reality and Buddha's teaching, which consists in understanding reality to free oneself from attachments (and then from pains).
Instead of "Buddhism" better use the word dhamma, because this one is more precise. There are many ways in Buddhism which are quite different from the Buddha's teaching. For this reason when you say " I do dhamma, you know you are talking about reality, about Buddha's teachings.
However, no matter what words are used, do not forget that the most important thing it is to practice immediately because the biggest vantage when we are young is that we can directly see things. Thereby we can easily understand reality. Getting older we easily may complicate matters for nothing. Sometimes we think hard. It is important to think over what you do but if you think too much you don't practice anymore.
No need to mull because there nothing easier than dhamma !