TDS (Total Dissolved Solids).

Detailed Explanation:
Measured as a percentage using a refractometer, TDS helps baristas evaluate brewing efficiency. 

Higher TDS usually means stronger coffee, but balance matters… too much can indicate over-extraction and bitterness. 

Specialty coffee typically ranges from 1.15%-1.35% TDS in drip coffee, while espresso sits around 8-12%. 

While home brewers may not measure TDS, understanding it helps refine brewing technique and consistency.