Citrus trifoliata L. (syn: Aegle sepiaria DC.; Bilacus trifoliata (L.) Kuntze; Citrus trifolia (L.) Thunb.; Citrus trifoliata monstrosa (T. Ito) Hiroe; Citrus trifoliata var. monstrosa T. Ito; Citrus triptera Desf.; Poncirus trifoliata (L.) Raf.; Poncirus trifoliata var. monstrosa (T. Ito) Swingle; Pseudaegle sepiaria (Candolle) Miquel; Pseudaegle trifoliata (L.) Makino) as per Catalogue of Life;
Poncirus trifoliata (L.) Raf., Sylva tellur. 143. 1838 (Syn: (≡) Citrus trifoliata L. (basionym); (=) Citrus trifoliata var. monstrosa T. Itô; (=) Poncirus trifoliata var. monstrosa (T. Itô) Swingle);
Central & S. China; Introduced into: Alabama, Argentina Northeast, Arkansas, Bulgaria, Delaware, Florida, Georgia, Illinois, Japan, Korea, Louisiana, Maryland, Mississippi, Missouri, North Carolina, Oklahoma, Pennsylvania, Romania, South Carolina, Tadzhikistan, Tennessee, Texas, Uzbekistan, Virginia, West Virginia as per POWO;
China (Anhui, Chongqing, S-Gansu, N-Guangdong, N-Guangxi, Guizhou, Henan, Hubei, Hunan, Jiangsu, NW-Jiangxi, S-Shaanxi, Shandong, S-Shanxi, Zhejiang), Taiwan (I), Japan (I), Korea (I), Tajikistan, Vietnam (I), Turkey (I) (NE-Anatolia (I)), India (I), USA (I) (Alabama (I), Arkansas (I), Delaware (I), Florida (I), Georgia (I), Louisiana (I), Maryland (I), Mississippi (I), North Carolina (I), Oklahoma (I), Pennsylvania (I), South Carolina (I), Tennessee (I), Texas (I), Virginia (I), West Virginia (I)), Argentina (I), Croatia (I) as per Catalogue of Life;

Trifoliate orange, Poncirus trifoliata (syn. Citrus trifoliata), is a member of the family Rutaceae, closely related to Citrus, and sometimes included in that genus, being sufficiently closely related to allow it to be used as a rootstock for Citrus. It differs from Citrus in having deciduous, compound leaves, and pubescent (downy) fruit.

It is native to northern China and Korea, and is also known as the Chinese Bitter Orange.
The plant is fairly hardy (USDA zone 5) and will tolerate moderate frost and snow, making a large shrub or small tree 4–8 m tall. Because of the relative hardiness of Poncirus, citrus grafted onto it are usually hardier than when grown on their own roots.
Poncirus trifoliata is recognisable by the large 3–5 cm thorns on the shoots, and its deciduous leaves with three (or rarely, five) leaflets, typically with the middle leaflet 3–5 cm long, and the two side leaflets 2–3 cm long. The flowers are white, with pink stamens, 3–5 cm in diameter, larger than those of true citrus but otherwise closely resembling them, except that the scent is much less pronounced than with true citrus. As with true citrus, the leaves give off a spicy smell when crushed.
The fruits are green, ripening to yellow, and 3–4 cm in diameter, resembling a small orange, but with a finely downy surface. They are very bitter, are not edible fresh, but can be made into marmalade, and when dried and powdered, they can be used as a condiment.
The fruits of Poncirus trifoliata are widely used in Oriental medicine as a remedy for allergic inflammation.[2] 
(From Wikipedia on 6.11.14)

Citrus for ID Tiruvannamalei, Tamil Nadu NAW-OCT-31 : 2 posts by 2 authors. Attachments (3).
Kindly identify this tree photographed in the herbal garden Tiruvannamalei, Tamil Nadu in September 2014.
Tree about 4-5 metres tall, the main trunks bearing thick spikes upto 8 mm in diameter and 4-5 cm in length.

Trifoliate orange, Poncirus trifoliata (syn. Citrus trifoliata)

SK1821 01 April 2019 : 4 posts by 3 authors. Attachments (10)
Location: Pame, Pokhara, Kaski Dt.
Date: 21 March 2019

Elevation: 827 m.

Habit : Looks like cultivated

Rutaceae. Any chance Citrus sp.  

I agree with …, for some Rutaceae member, may be some Citrus species.

Citrus trifoliata L.
Syn : Poncirus trifoliata (L.) Raf.
I guess it is introduced in Nepal.
Thank you … for the cue !