GG5: 【数】 a parameter.
中辞典: 【数】 a parameter.
ルミナス: ditto
Daijjisen: 変数の間の関数関係を、間接に表すために用いる変数。関数x=f(t)とy=g(t)とからｘとｙとの関数関係が定まるときのｔのこと。パラメーター。径数。母数。助変数。

Despite the approach taken by the JEs, I agree the simple use of "parameter" is misleading.

パラメータ	1206116	99.6%
媒介変数	5445	0.4%

https://kotobank.jp/word/媒介変数-112728
daijs
変数の間の関数関係を、間接に表すために用いる変数。関数x＝f（t）とy＝g（t）とからxとyとの関数関係が定まるときのtのこと。パラメーター。径数。母数。助変数。


引数	675369	73.8%
引き数	29709	3.2%
パラメーター	176358	19.3%
母数	27309	3.0%
媒介変数	5445	0.6%  <------ oof
径数	1099	0.1%
助変数	398	0.0%

https://ja.wikipedia.org/wiki/媒介変数
数学における媒介変数（ばいかいへんすう）、 助変数（じょへんすう、英: auxiliary variable; 補助変数）、パラメータ（英: parameter[注 1]; 母数、径数[注 2]）は


https://www.statisticssolutions.com/dissertation-resources/descriptive-statistics/mediator-variable
A mediator variable is the variable that causes mediation in the dependent and the independent variables. In other words, it explains the relationship between the dependent variable and the independent variable.

https://en.wikipedia.org/wiki/Mediation_(statistics)
 a third hypothetical variable, known as a mediator variable (also a mediating variable, intermediary variable, or intervening variable).

You can google 
"mediator variable" "媒介変数"
or
"mediator variable"　日本語
to verify that this is a thing... 

Worked backwards from the definition from daijs to find this.

This appears to be a "parameter" in a very limited sense (though "parameter" is a common gloss). Wikipedia has several different pages for "parameter" (generic, math, computer science, statistics, linguistics...).  it's an overloaded term.

This term probably shouldn't be used by anyone who doesn't know *exactly* what they are doing.  That's why I put "parameter" as the second gloss.

@@ -11,0 +12,4 @@
+<xref type="see" seq="1102630">パラメータ</xref>
+<xref type="see" seq="1102630">パラメータ</xref>
+<field>&math;</field>
+<gloss>mediator variable</gloss>