der Schweiß
sweat
The salty, clear fluid secreted by the sweat glands of the skin to cool the body.
Nach dem Joggen lief ihm der Schweiß von der Stirn.
After jogging, sweat was running down his forehead.

sweat
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sweat
The salty, clear fluid secreted by the sweat glands of the skin to cool the body.
Nach dem Joggen lief ihm der Schweiß von der Stirn.
After jogging, sweat was running down his forehead.

'Schweiß' (sweat) is an uncountable noun and is almost always used in the singular. So you don't say 'viele Schweiße' (many sweats), but 'viel Schweiß' (a lot of sweat).
Bei dieser Hitze verliert man viel Schweiß.
In this heat, you lose a lot of sweat.
For the physical reaction, there are fixed expressions like 'in Schweiß ausbrechen' (to break out in a sweat), 'in Schweiß gebadet sein' (to be drenched in sweat), or 'kalter Schweiß' (cold sweat). These describe sudden or heavy sweating.
Als er die Spinne sah, brach er in kalten Schweiß aus.
When he saw the spider, he broke out in a cold sweat.
In the figurative sense, 'Schweiß' (sweat) is often connected with effort and its results. Typical expressions are 'etwas kostet Schweiß und Mühe' (something takes sweat and effort) or the well-known phrase 'Blut, Schweiß und Tränen' (blood, sweat, and tears).
Der Aufbau der Firma hat ihn viel Schweiß gekostet.
Building the company cost him a lot of sweat.
You can recognize the literal meaning from contexts like sports, heat, or physical reactions (e.g., fear). The figurative meaning is found in contexts involving work, projects, and achieving goals.
Beim Saunabesuch kommt man ordentlich ins Schwitzen.
When you visit the sauna, you really start sweating.
water
'das Wasser' and 'der Schweiß' can both refer to liquid on the body. 'Schweiß' is the exact liquid that comes from the skin when you are hot, exercising, or making an effort. 'Wasser' is much more general and may only describe that something is wet or looks like water. Sometimes people also say figuratively that water is running down someone's back. Learners may confuse the words because sweat is clear and liquid. If you mean the body's reaction, 'Schweiß' is the correct and more natural word.

Beim Training stand Max der Schweiß auf der Stirn.
During training, sweat stood on Max's forehead.
Nach dem langen Lauf lief ihm das Wasser über das Gesicht.
After the long run, water was running down his face.