Yukti-ṣaṣtikā 

‘Sixty [Verses] of Reasoning’, a work by Nāgārjuna discussing the relationship between absolute truth (paramārtha-satya) and relative truth (saṃvṛti-satya) in terms of the doctrine of Dependent Origination (pratītya-samutpāda).

A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)