tapas is a Sanskrit term which translates as "heat". It is the 'heat' (i.e.passion, zeal or "energised enthusiasm" - to use Crowley's term) generated by the self-disciplined application of sadhana. Tapas is often understood in the context of performing 'austerities' in the sense of self-denial or self-sacrifice. However, Tantric practitioners generally had a low opinion of the value of practising 'austerities' in this manner:
"If people could get liberated by smearing themselves with dust and ashes, are all the country folk, who live amidst dust and ashes liberated?" "Animals like pigs bear the winter cold and summer heat and for them food fit or unfit is alike, are they Yogis thereby?"
The Kulanarva Tantra
Tapas can be understood as the 'heat' generated from disciplined spiritual practice - particularly those practices that help us free ourselves from Kleshas, attachments and neurotic behaviour. There is also the matter of keeping a balanced perspective on practice. Practices such as fasting or keeping silence have their place, but can become inappropriate (or even counter-effective) if taken to extremes.