The latitude is the angular distance of a point on the Earth’s surface, north or south, of the Equator as measured from the:

Latitude is defined as the angular distance of a point on the Earth's surface, north or south of the Equator, measured in degrees from the centre of the Earth [t2]. While the Equator serves as the reference plane (0° latitude), the actual angle is subtended at the Earth's center [t2][t4]. Specifically, geocentric latitude is the angle between the equatorial plane and a line connecting the point to the Earth's center [t1][t2]. In contrast, geodetic latitude—the standard used in mapping—is the angle between the equatorial plane and a line normal to the surface [t1][t5]. However, in general geographic definitions, the measurement is fundamentally described as an angular distance from the center [t4]. This system allows for the division of the Earth into the Northern and Southern Hemispheres, reaching 90° at the poles [t4][t7].

Sources

1. [1] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 2: The Earth's Crust > To calculate the latitude of a place from the Sun's elevation, at a certain part of the year > p. 9
2. [2] https://oceanservice.noaa.gov/facts/latitude.html
3. [3] https://en.wikipedia.org/wiki/Geodetic_coordinates