Deliverance.
The word used in 18:19 is azkaa. It is the superlative of zakeey or zakeeyah, which means good or pure. It means more pure or most pure. Like Akbar is the superlative of kabeer, meaning most kabeer or more kabeer (kabeer has so many meanings I am deciding not to translate all them in order to be fair to the word). Zakaah is not used in 18:19.
Saba,
I have a lot of issues with Lane's lexicon, but I won't go there because for general purposes and when better is not available it is fine. I generally refer to Lisanul-Arab and other completely Arabic lexicons which are older and more comprehensive than Lane's lexicon can ever be. Why he didn't just translate Taj Al-'Arous or Lisanul-Arab is beyond me.
What you are not realizing here is that Zakaah as charity clearly has its origin in post-Quranic Islamic opinion. This is where Lane fails and where earlier and more authoritative Classical Arabic lexicons succeed, as they are interested in Classical Arabic as a whole, and in detailing what is post-Quranic and Islamic and what is not, many times even referring to the pre-islamic poetries to show meanings that some meanings were clearly used in pre-Islamic times but generally fell out of use after the Quran. Take Mu'jam Al-faadh Al-Quran's comment that raaki3 also had a meaning of monotheist and one who did not worship idols among the pre-islamic Arabs. Lane's sources on many words are clearly Islamic and not linguistic in nature.
This means that anything clearly Islamic in origin, i.e., post-Quranic cannot be read into the Quran because the Quran's language is naturally pre-Islamic and pre-Quranic in origin as it borrowed the language of the pre-Islamic and pre-Quranic Arabs. Again, this means that ANY meaning of a word with an Islamic and post-Quranic origin cannot be read into the Qur'an.
Lisanul-Arab first has Zakaah as righteousness/correctness (Al-Salaa7) and has Az-zakaah as zakaatul-maal, i.e. zakaah as zakaah of the wealth (this is already logically problematic). He goes on to say that this is known and that it is purification (tat-heer) of one's wealth. He adds that it is "what you take out from your wealth in order to purify it." I don't know if you see the logical flaws here but the first is in explaining al-zakaah using the same word again, and stating that it is zakaah of one's wealth, which is then explained as purifying one's wealth. This already intimates that Zakaah as purity is the stronger and more original meaning.
It also begs questions such as did the pre-Islamic Arabs have a concept of purifying wealth or setting aside some of one's wealth in order to purify it? And is this even a Quranically valid concept? Does the wealth we earn morally and within Quranic guidelines need to be purified?
What is mentioned next is a partial phrase from 9:103.
خذ من أموالهم صدقة تطهرهم و تزكيهم بها و صلي عليهم إن صلاتك سكن لهم و الله سميع عليم
"Take, [O, Muhammad], from their wealth a charity by which you purify them and cause them increase, and invoke [ Allah 's blessings] upon them. Indeed, your invocations are reassurance for them. And Allah is Hearing and Knowing."
The part that is mentioned is و تزكيهم بها " wa tuzakkeehim bihaa "
It says that they say this means تُطهِّرُهم tutahhiruhum: it purifies them
Tutahhiruhum is actually mentioned just previous to 'wa tuzakeehim' in the above passage. In my opinion understanding tazkiyah as tat-heer in THIS PARTICULAR context would be a little too redundant. I would use some of its other meaning which we will see below.
Then it says that Zakaah means صفوةُ الشيء which is 'the purity of a thing.
Zakaah is afterwards again defined as righteous or correct acts i.e. العمل الصالح
Then we have another explanation for what is taken out for the poor being part of their rights (min huqooqihim), or what they are entitled to (the use of rights here is a dead give away that this is a shar'iah or pos-Quranic Islamic opinion) being called Zakaah because it purifies the wealth, makes it produce, is a form of making things right/correct, i.e. rectification and is a form of growth. The original statement is below:
قيل لما يُخْرَج من المال للمساكين من حقوقهم زَكاةٌ لأَنه تطهيرٌ للمال وتَثْميرٌ وإِصْلاحٌ ونماء،
The nail in the coffin is this statement:
وأَصل الزكاة في اللغة الطهارة والنَّماء والبَركةُ والمَدْح وكله قد استعمل في القرآن والحديث
"The original meaning of Zakaah in the Classical Arabic language is purity, growth, blessing and commendation/praise/acclaim."
This is also essentially the first sentence in Mu'jam Al-faadh Al-Quran
It says that "the original meaning of Zakaah is growth that attains God's blessing and is used with worldly affairs and afterworldly affairs. It is said that what was planted zakaa (past tense, singular masculine) and yazkoo (present tense, singular, masculine) if it attains growth and blessing. And the verse that says أيها ازكا طعاما ('Ayyuhaa azkaa ta3aaman i.e 'which is better to eat.' Here azkaa is the superlative of zakeey) and indicates what is lawful and has no unwholesome consequences. From this comes Zakaah as what men owe to God for the poor and it is called this because it is seeking blessing or purification of the soul or its growth with what is good and with blessings."
Both Lisanul-Arab and Mu'jam Al-faadh Al-Quran state that the original meaning of the word in Classical Arabic is growth, purity, blessing and praise (only Lisanul-Arab says praise). Additionally they use Al-Zakaah as charity in connection to the rights (huqooq) of those in need and what we owe to God (literally the rights of God, haqqullah) making this meaning clearly Islamic and therefore post-Quranically theological in meaning.
Zakaah as charity is in essence a post-Quranic, Islamic nickname for charity (saduqah/sadaqah) because of how it implies growth for others and for our souls, because it is good, productive, purifies us and brings God's blessing on us.
The reasons for Zakaah being taken as a nickname for charity can be found in 9:103 where sadaqah is said to be tazkiyah for us; zakaah (purity) and tazkiyah (purification) being essentially interchangeable. I'm sure in a number of hadeeth which also have been taken as a basis for nicknaming sadaqah Zakaah.
I hope this has helped.