These are compound words.

"Indra + ayudha" means literally "the weapon of Indra" (Indra was the thunder deity in the Vedic mythology, similar to Greek Zeus).

"Indra-cApa" means "the bow of Indra".
(note the "A" in cApa which means a special long "a" usually denoted as "a" with a dash above it).

These are, probably, not the only words in Sanskrit for "rainbow", but they are options for translations. If I have time I will try to provide use with more options later...

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Note added at 42 mins (2004-10-15 14:22:20 GMT)
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btw, cApa is read like \"ch-A-p-a\"

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Note added at 1 day 24 mins (2004-10-16 14:03:49 GMT)
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To Jaswinder: ok, I do not know how do you *speak* it (I agree that short a\'s at the end of many words in Sanskrit may be optional), but there is an accepted system of writing Sanskrit word in English. \"Inder\" is obviously not the accepted way, that\'s all.

Here are just two links on Sanskrit-English dictionaries in my support:

http://www.mendonet.com/jyotii/dictnry/sanskrit.htm

indrachaapa = (m) rainbow
indradhanuH = (m) rainbow

http://students.washington.edu/prem/mw/i.html

indracāpa
○cāpa m. n. Indra\'s bow, the rainbow

indrakārmuka
○kārmuka n. rainbow

indradhanus
○dhanús n. Indra\'s bow, the rainbow AV. xv, 1, 6 (NOTE: dhanuS, not dhanuSH)

indrāyudha
indrâyudha n. \'Indra\'s weapon\', the rainbow MBh. VarBṛS. Ragh.
• diamond L.
• m. a horse marked with black about the eyes
• (ā), f. a kind of leech (marked with rainbow tints) Suśr.
• -maya mfn. consisting of rainbow Kād.
• -śikhin m. N. of a Nāga