A depth below Earth's surface which stays at the same temperature regardless of surface seasonal and daily temperature variations.

Strictly speaking the surface temperature variations propagate down the Earth infinitely and it goes down to measurement accuracy to assess where exactly the surface temperature variations decay below the measurement threshold  $//<![CDATA[ \begin{array}{l}\delta T_{\rm cut}\end{array} //]]>$, which is usually varies between the most common value of  $//<![CDATA[ \begin{array}{l}\delta T_{\rm cut} = 0.1 \, {\rm °C}\end{array} //]]>$ down to the most accurate  $//<![CDATA[ \begin{array}{l}\delta T_{\rm cut} = 0.001 \, {\rm °C}\end{array} //]]>$.

In this sense, the Neutral Temperature Layer is more of an engineering concept rather than true physical property of the subsurface medium.

See Also

Geology / Geothermal Temperature Field 

[ Geothermal Temperature Field  ] [ Geothermal Temperature Profile ] 

[ Neutral Temperature Layer @model ]