is-

1.
variant of iso- before a vowel:
isallobar.
British Dictionary definitions for is-

is-

combining form
1.
variant of iso- isentropic

iso-

combining form
1.
equal or identical: isomagnetic
2.
indicating that a chemical compound is an isomer of a specified compound: isobutane, isocyanic acid
Word Origin
from Greek isos equal
is- in Medicine

is- pref.
Variant of iso-.