The ratio of water to cementitious materials in a concrete or mortar mixture, typically expressed by weight. The water-cement ratio is a critical factor in determining the strength, durability, and workability of the mixture. Lower water-cement ratios generally result in stronger, more durable concrete or mortar, but may also reduce workability.
http://www.wikidata.org/entity/Q1413436, https://en.wikipedia.org/wiki/Water%E2%80%93cement_ratio