The Concise Oxford Dictionary of World Religions (1388 page)

(Skt., ‘world’).
Hinduism
The domains or regions which make up the cosmos. In origin there were three lokas (
triloka
), locations inhabited by beings appropriate to them: earth, atmosphere, and ‘the yonder world’ (of the gods, the sun, the moon, and the stars). But these were early related to domains pertaining to salvation (and its opposite). Thus
svar
, ‘sky’, is already synonymous in the
Vedas
with
svarga
, ‘heaven’, so that another triad was produced of svarga-loka,
bh
mi
- (earth), and
p
t
la
- (underworld or hell).
Buddhism
The primary sense is analytic, in which loka is the ‘habitat’ of gods and human beings. In these contexts, loka is explained as all the perceptible world, i.e. all that which comes within the spheres of the senses. Loka in its cosmographic sense includes the entire cosmos. A
lokadh
tu
is a smaller unit within the loka, a unit which may be described as a solar system. Thus, the loka consists of myriads of such solar systems. Therefore, in its immensity, the loka is unlimited. It is not possible, therefore, to reach the end of the loka by travelling and its immensity cannot be grasped by thinking either. Hence,
lokacint
is one of four unthinkables according to
Anguttara Nik
ya
(2. 80).
Buddhism also has its equivalent to the triloka of Hinduism (in P
li,
trailokya
or
traidh
tuka
). They are

(i) k
maloka, the domains of desire and attachment, including those of hell (
naraka
), humans, animals, the
devas
, and the
asuras
;
(ii) r
paloka, the domain of form without desire, the gods in the
dhy
na
heaven, attained through the four dhy
nas;
(iii) ar
paloka, the domain of formlessness. They may also be known as K
madhatu, etc