It is the air temperature at which condensation of moisture begins when the air is cooled.
OR
It is the saturation temperature of air corresponding to the partial pressure of water vapor.
OR
It is the temperature of moist air saturated at the same pressure & same humidity ratio.

When the RH is 100%, then the air is said to be Saturated.
So, @ Saturation DBT = WBT = DPT

Partial pressure of water vapour ( $p_{w}$ ) (VP)

When you have DBT & WBT, follow the instructions Humidity Ratio OR Specific Humidity and get the Partial pressure of water vapour (VP) in psia.

When you have DBT & RH, follow the instructions Humidity Ratio OR Specific Humidity and get the Partial pressure of water vapour (VP) in psia.

Assume the calculated value $p_{w}$ = 0.303 psia @ 75°F DBT, 68° WBT, & 10ft from sea level (OR) 75° F, 70.5% RH, & 10ft from sea level.

Calculation for Dew point temperature (DP)

$\alpha = \ln(p_{w})$

For above 32° F

$t_{dp} = C_{14}$ $+$ $C_{15}*\alpha + C_{16}*\alpha^2$ $+$ $C_{17} * \alpha^3 + C_{18}*p_{w}^{0.1984}$

For below 32° F

$t_{dp} = 90.12 + 26.142 * \alpha + 0.8927 * \alpha^2$

Where,
$t_{dp}$ - Dew point temperature in °F
$p_{w}$ - Partial pressure of water vapour (VP) in psia
C14, C15, …, C18 are constants and their values are
100.45, 33.193, 2.319, 0.17074, 1.2063
respectively

$\alpha = \ln(0.303) = -1.194$

$t_{dp} = 100.45$ $+$ $33.193*(-1.194) + 2.319*(-1.194)^2$ $+$ $0.17074 * (-1.194)^3 + 1.2063*0.303^{0.1984}$
= 64.78° F

Dew Point Temperature (DPT)

Partial pressure of water vapour (pwp_{w}p​w​​) (VP)

Calculation for Dew point temperature (DP)

Partial pressure of water vapour ( $p_{w}$ ) (VP)