About the word "estimand":
Apparently of mid-20th century coinage from Latin aestimandum, gerundive of aestimo (“I value", "I estimate”).
estimand (plural estimands)
(statistics) that which is being estimated.
[definition edited for format]
Some words ending in -and and -end have the same sense as this one - they denote something that receives the action of the preceding verb form, or something that should receive it.
multiplicand, addend, augend, solvend, adherend, dividend...
Whether it's -and or -end depends on the class of Latin verb it is derived from.
As for the collocation causal estimand, it seems to mean an estimate of the degree of causality in a relation.
Google Books shows the term in multiple uses:
Google Books causal estimand
Two of the book titles are "Causal Inference" and "Statistics and Causality".
While this isn't an early English learner's term, the information about the use of the -and and -end endings may be of interest. Note, though, that most English words that end with these letters aren't examples of the same usage.