Like there is a meaning of the word Shiva which is explained here, what is the meaning of Indra?
There are different meaning of the word Indra is given by Yaska in Nirukta 10.8.
इन्द्र इरां दृणातीति वाँ।
इरां दद्तीति वा।
इरां दधातीति वा ।
इरा दारयतैं इति घा।
इरा धारर्थत इति वा ।
इन्दवे द्रर्वतीति वा ।
इन्दी रमत इति वा ।
इन्धे भूतानीति वा ।
तद्यदेनं प्राणैः समैनैधंस्तदिन्द्रस्येन्द्रत्वैम् ।।
इति विज्ञायैते ।
इदं करणादित्याग्रयणैः ।
इदं दर्शनादित्यौपमन्यवः ।
इन्दञ्छत्रूणां दारयितैा वा ।
द्रावयिता वा ।
अादरयिता च यज्वनामं ।
तस्यैषा भवति ॥ ८ ॥
Indra is (so called because) he divides food (irâ + dṛ), or he gives food (irâ + dâ), or he bestows food (irâ + dhâ), or he sends food (irâ + dâraya), or he holds food (irâ + dhâraya), or he runs for the sake of soma (indu + dru), or he takes delight in soma (indu + ram), or he sets beings on fire (√indh). It is known: that because they animated him with vital breaths, that is the characteristic of Indra,” “He is (so called) from doing everything (lit. this), says Agrayana. He is (so called) from seeing everything (idam. + dṛś), says Aupamanyava. Or the word is derived from the verb) id., meaning to be powerful, i.e. being powerful he tears the enemies asunder, or puts them to flight. Or he honours the sacrificers. The following stanza is addressed to him.
Satapatha Brahmana 6:1:1:2 defines Indra as follows:
This same vital air in the midst doubtless is Indra. He, by his power (indriya), kindled those (other) vital airs from the midst; and inasmuch as he kindled (indh), he is the kindler (indha): the kindler 3 indeed,--him they call 'Indra' mystically (esoterically), for the gods love the mystic. They (the vital airs), being kindled, created seven separate persons (purusha).
what is the meaning of Indra?
The exact meaning of word Indra is given in Aitareya Upanishad –Khanda 3 –Chapter 1 –Verse 14. - Page no 32-33
तस्मादिन्द्रो नामेदन्द्रो ह वै नाम | तमिदन्द्रं सन्तमिन्द्र इत्याचक्षते परोक्षेण |
परोक्षप्रिया इव हि देवा: परोक्षप्रिया इव हि देवा: ||14||
tasmAdidandro nAmedandro ha vai nAma . tamidandra.n santami.ndra ityAchakShate parokSheNa
parokShapriyA iva hi devAH parokShapriyA iva hi devAH .. 14..
Therefore He is called Idandra. Idandra is the well known name of the God.Him ,though Idandra , they call Indra indirectly, for the gods are fond of being incognito , as it were.
This is Shankara’s commentary -
: As he saw the all permeating “Brahman” directly as an object , as “ idam” OR this therefore , the Paramatman is called Idandra.The lord is well known in the world as Idandra. The knowers of Brahman called the “Brahman” who is really Idandra ,by the name of Indra , a word denoting some object beyond the range of vision ; So that it may be freely talked about , by afraid of calling him by his real name as he is regarded as most venerable. For the Devas (deities) are fond of assuming the names denoting invisible objects. Much more so ,should be therefore , the lord of all the deity of all the deities.
From the commentary we come to know that “ Indra “ is actually a short form of name “ Idandra”. Means , a word denoting some object beyond the range of vision ; and the lord of all the deity of all the deities.
Indra is the king of the Hindu gods. According to Hinduism, Indra is the god of heaven, war and thunder and storm. Indra is also worshiped by followers of Buddhism and Jainism.
Indra means "possessing drops of rain" from Sanskrit इन्दु (indu) meaning "a drop" and र (ra) meaning "acquiring, possessing". Indra is the name of the ancient Hindu warrior god of the sky and rain. He is the chief god in the Hindu text the Rigveda. (Wiki source and Yogapedia).