The letter immediately preceding the last letter of a word is called penultimate (upadhā).,
This defines उपधा or penultimate. Thus in the words पच्, भिद्, बुध्, वृत् the letters अ, इ, उ, and ऋ are उपधा.
The word 'upadha' occurs in sutra 7.2.116.,
अलोऽन्त्यात् पूर्व उपधा
Adhyāya 1 · Pāda 1 · Rule 65