What is a line that connects points on a map that have the same temperature called?

An isotherm (from θέρμη or thermē, meaning 'heat') is a line that connects points on a map that have the same temperature. Therefore, all points through which an isotherm passes have the same or equal temperatures at the time indicated. An isotherm at 0 °C is called the freezing level. The term was coined by the Prussian geographer and naturalist Alexander von Humboldt, who as part of his research into the geographical distribution of plants published the first map of isotherms in Paris, in 1817.

The other options' meanings:

An isoneph is a line indicating equal cloud cover.

An isobront is a line drawn through geographical points at which a given phase of thunderstorm activity occurred simultaneously.

An isohume is a line of constant relative humidity.