Glossary:

frequency offset:

The difference between the ntpd calculated frequency and the local system clock frequency (usually in parts per million, ppm)

jitter, dispersion:

The short term change in a value. NTP measures Local Time Jitter, Refclock Jitter, and Server Jitter in seconds. Local Frequency Jitter is in ppm or ppb.

kurtosis, Kurt:

The kurtosis of a random variable X is the fourth standardized moment and is a dimension-less ratio. ntpviz uses the Pearson's moment coefficient of kurtosis. A normal distribution has a kurtosis of three. NIST describes a kurtosis over three as "heavy tailed" and one under three as "light tailed".

ms, millisecond:

One thousandth of a second = 0.001 seconds, 1e-3 seconds

mu, mean:

The arithmetic mean: the sum of all the values divided by the number of values. The formula for mu is: "mu = (∑x_i) / N". Where x_i denotes the data points and N is the number of data points.

ns, nanosecond:

One billionth of a second, also one thousandth of a microsecond, 0.000000001 seconds and 1e-9 seconds.

percentile:

The value below which a given percentage of values fall.

ppb, parts per billion:

Ratio between two values. These following are all the same: 1 ppb, one in one billion, 1/1,000,000,000, 0.000,000,001, 1e-9 and 0.000,000,1%

ppm, parts per million:

Ratio between two values. These following are all the same: 1 ppm, one in one million, 1/1,000,000, 0.000,001, and 0.000,1%

‰, parts per thousand:

Ratio between two values. These following are all the same: 1 ‰. one in one thousand, 1/1,000, 0.001, and 0.1%

refclock:

Reference clock, a local GPS module or other local source of time.

remote clock:

Any clock reached over the network, LAN or WAN. Also called a peer or server.

time offset:

The difference between the ntpd calculated time and the local system clock's time. Also called phase offset.

σ, sigma:

Sigma denotes the standard deviation (SD) and is centered on the arithmetic mean of the data set. The SD is simply the square root of the variance of the data set. Two sigma is simply twice the standard deviation. Three sigma is three times sigma. Smaller is better.
The formula for sigma is: "σ = √[ ∑(x_i-mu)^2 / N ]". Where x_i denotes the data points and N is the number of data points.

skewness, Skew:

The skewness of a random variable X is the third standardized moment and is a dimension-less ratio. ntpviz uses the Pearson's moment coefficient of skewness. Wikipedia describes it best: "The qualitative interpretation of the skew is complicated and unintuitive."
A normal distribution has a skewness of zero.

upstream clock:

Any server or reference clock used as a source of time.

µs, us, microsecond:

One millionth of a second, also one thousandth of a millisecond, 0.000,001 seconds, and 1e-6 seconds.