The word(s) "illegal" appears 42 time(s) in 36 verse(s) in Quran in Mohsin Khan translation.

(21) "And befool them gradually those whom you can among them with your voice (i.e. songs, music, and any other call for Allah's disobedience), make assaults on them with your cavalry and your infantry, share with them wealth and children (by tempting them to earn money by illegal ways - usury, or by committing illegal sexual intercourse), and make promises to them." But Satan promises them nothing but deceit.

(22) And those who guard their chastity (i.e. private parts, from illegal sexual acts)

(23) The fornicatress and the fornicator, flog each of them with a hundred stripes. Let not pity withhold you in their case, in a punishment prescribed by Allah, if you believe in Allah and the Last Day. And let a party of the believers witness their punishment. (This punishment is for unmarried persons guilty of the above crime, but if married persons commit it (illegal sex), the punishment is to stone them to death, according to Allah's Law).

(24) Verily, those who like that (the crime of) illegal sexual intercourse should be propagated among those who believe, they will have a painful torment in this world and in the Hereafter. And Allah knows and you know not.

(25) O you who believe! Follow not the footsteps of Shaitan (Satan). And whosoever follows the footsteps of Shaitan (Satan), then, verily he commands Al-Fahsha' [i.e. to commit indecency (illegal sexual intercourse)], and Al-Munkar [disbelief and polytheism (i.e. to do evil and wicked deeds; and to speak or to do what is forbidden in Islam)]. And had it not been for the Grace of Allah and His Mercy on you, not one of you would ever have been pure from sins. But Allah purifies (guides to Islam) whom He wills, and Allah is All-Hearer, All-Knower.

(26) Tell the believing men to lower their gaze (from looking at forbidden things), and protect their private parts (from illegal sexual acts). That is purer for them. Verily, Allah is All-Aware of what they do.

(27) And tell the believing women to lower their gaze (from looking at forbidden things), and protect their private parts (from illegal sexual acts) and not to show off their adornment except only that which is apparent (like both eyes for necessity to see the way, or outer palms of hands or one eye or dress like veil, gloves, head-cover, apron, etc.), and to draw their veils all over Juyubihinna (i.e. their bodies, faces, necks and bosoms) and not to reveal their adornment except to their husbands, or their fathers, or their husband's fathers, or their sons, or their husband's sons, or their brothers or their brother's sons, or their sister's sons, or their (Muslim) women (i.e. their sisters in Islam), or the (female) slaves whom their right hands possess, or old male servants who lack vigour, or small children who have no sense of feminine sex. And let them not stamp their feet so as to reveal what they hide of their adornment. And all of you beg Allah to forgive you all, O believers, that you may be successful.

(28) And those who invoke not any other ilah (god) along with Allah, nor kill such person as Allah has forbidden, except for just cause, nor commit illegal sexual intercourse - and whoever does this shall receive the punishment.

(29) O wives of the Prophet! Whoever of you commits an open illegal sexual intercourse, the torment for her will be doubled, and that is ever easy for Allah.

(30) Verily, the Muslims (those who submit to Allah in Islam) men and women, the believers men and women (who believe in Islamic Monotheism), the men and the women who are obedient (to Allah), the men and women who are truthful (in their speech and deeds), the men and the women who are patient (in performing all the duties which Allah has ordered and in abstaining from all that Allah has forbidden), the men and the women who are humble (before their Lord - Allah), the men and the women who give Sadaqat (i.e. Zakat, and alms), the men and the women who observe Saum (fast) (the obligatory fasting during the month of Ramadan, and the optional Nawafil fasting), the men and the women who guard their chastity (from illegal sexual acts) and the men and the women who remember Allah much with their hearts and tongues Allah has prepared for them forgiveness and a great reward (i.e. Paradise).