For your purposes, (a dictionary of key terms, unfortunately there is no government publication. However, one good hack you can consider is to look at the Section 3 of the General Clauses Act, 1897. The terms defined under this Act will govern the meaning in all the central/federal legislations and rules unless those legislations specifically define these terms. The terms defined under General Clauses Act include terms such as affidavit, immovable property, person etc. (essentially terms similar to the ones used in your dictionary). Here is the link; hope you find it useful (https://www.indiacode.nic.in/show-data?actid=AC_CEN_3_20_000...)

p.s.- https://www.indiacode.nic.in is also a government website where you can find copies of legislations and rules.