There are many ways to specify thread thickness. Thankfully the standards are well-defined with some units more common than other.
In Europe the most common ones are:
Tex is actually an SI unit which can be used on anything from tiny fibers to huge cables.
1 tex = 1 gram per kilometer.
So one kilometer of a thread of 1 tex weights 1 gram.
On fibers it's more common to use dtex (desitex) or 1/10 tex.
1 dtex = 0.1 tex, same way as 1 dl (desiliter) = 0.1 liter.
In other words 1 dtex = 0.1g per kilometer or 1g per 10km (10,000m).
Normal thickness of sewing threads are 30 tex (30 grams per kilometer).
Nm is somehow the opposite of tex. Nm means how many meters you have in 1 gram of thread.
Nm 50 translates to 50m in 1g thread.
Quite often you'll see Nm specified as a fraction, i.e. Nm 100/2.
The latter number represents the number of yarns the thread is made of.
A thread is normally made by twisting together smaller yarns. Two are quite common on normal and thin threads while 3 are common on thicker threads.
Below is a photo of a thread made of 3 yarns.
In other words, Nm tells both the thickness and the make of the thread.
Nm 100/2 and Nm 150/3 have the same thickness (equals to 50m per gram), men they consists of respectively two and three smaller yarns.
Normal thickness sewing threads are Nm 66/2 (33m per gram, to yarns).
This thread consist of 3 smaller yarns.
Ticket is a simplified version of Nm.
It's often called just "number" and shortened as "tkt." or "no.".
Thread number equals to Nm/3 regardless of how many yarns they're actually made of.
Number 80 equals to Nm 80/3 (80 / 3 = 26,7m per gram) even though it may consist of only 2 yarns.
You can also say 80 meters weights 3 grams.
This is the most common way to express thread thickness.
Normal thickness sewing threads have number 100 (or tkt 100) (100 meters weights 3 grams).
Thread number 50 is twice as thick as number 100.
While tkt 200 is half the thickness of tkt 100.