I have finished writing one paper, but have not submitted it yet to a journal. I am working on a related but not identical subject, though many of the definitions and theorems are same (but they are true in a very different context compared to the first paper). Now, is it okay to use the same figures that I had used for illustrations in my first paper? What about the theorems and definitions? I feel like changing the theorem statements is not worth spending time on, but is this correct?
Most journals require that the submitted manuscript has not been submitted total or partially to any other journal. Of course different policies apply to open access journals. If I were you I would check the journal's policy regarding this issue and contact the editors if necessary to get their opinion (probably they will require details on what will be duplicated). To the very least discuss this with your supervisor. You can find yourself in a very unpleasant position if you are not careful (not to mention having your papers rejected), as indicated by @Enthusiastic Student