Tapas

Tapas, which means something in between 'heat' and 'perseverance,' is interesting because it is (as a word) is often used somewhat outside of the context of the niyamas in various ways, such as to describe a more physical aspect of yoga, the heat generated in the body by some kinds of practice, or in a more spiritual sense the 'burning' off of negative energies and karma through good works or personal development and dedication.

As a principle of self-control (the niyama context), tapas might be understood better as the effort put forth in developing one's self. In that sense, it's connected to phrases like, 'fiery determination.' Tapas is the quality that would take one from simply daydreaming about something, to willing it into being through effort and action. It is the name of the effort particular to making a dream real.

Interestingly, it's worth noting that over-doing effort can burn us out and lead straight towards being overly attached to desires and their outcomes. The solution to that is to develop santosha, contentment, concurrently, to keep from 'burning' ourselves (I suppose an analogy might be putting extra effort into massaging someone, and then wearing out one's own hands or hurting the other person). Of course, tapas then balances santosha in turn, helping to avoid the previously mentioned shadow of passiveness.
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"There are two parallel tasks in spiritual life. One is to discover selflessness, the other is to develop a healthy sense of self. Both sides of that apparent paradox must be fulfilled for us to awaken."