Withanolides
The class of steroidal lactone compounds in ashwagandha root believed to drive its stress-reduction and other studied effects. Standardized ashwagandha extracts guarantee a minimum withanolide percentage; plain root powder and unstandardized extracts do not.
Example
The most-cited ashwagandha trial (Chandrasekhar et al. 2012) used a high-concentration, full-spectrum extract standardized specifically for withanolide content.