Definitions: Sattva

Sattva सत्त्व

• being , existence , entity , reality
• true essence , nature , disposition of mind , character
• spiritual essence , spirit
• the quality of purity or goodness regarded in the sāṃkhya darśana (or philosophy) as the highest of the three guṇa-s as it renders a person true , honest , a thing pure , clean

Discussion
This sattva is often used within the term of the triguṇa ( 3 guṇa-s). People may say ' that is very sattvic'. This word is quite profound as it means a level of purity unmatched or the can be delivered on the level of bhu (भु) or 'produced' creation. Sattva at its purest is Being Itself, stainless Being.
Another word for sattva is sattā सत्ता, which also means existence, Being. When one inspects both words sattā & sattva we can see both share a common bond of sat सत्. What is sat? That which really is, or existence, essence, the true being or really existent i.e. Truth.

If we inspect all 3 words we easily see the connection to 'sa' - a noun for for viṣṇu or śiva; 'sa' is also a noun for knowledge and meditation. Those tools or techniques ( knowledge, meditation) that take you to 'truth' , to viṣṇu ,śiva or satī¹.

References
A Concise Dictionary Of Indian Philosophy -John Grimes
Monier Williams Sanskrit-English Dictionary (2008 revision)
Extending the sati conversation , see HDF post http://www.hindudharmaforums.com/showthread.php?t=3651