DHIKR or ZIKR
Dhikr or Zikr is a special type of Islamic worship which means remembrance of Allah سبحانه و تعالى Almighty. Remembering Allah سبحانه و تعالى(dhikr) is one of the noblest deeds that a Muslim can do. Dhikr is not only verbal; rather we may remember Allah سبحانه و تعالىin our hearts, on our lips and in our physical actions.
Shaykh ‘Abd al-Rahmaan ibn Sa’di رحمت اللهsaid: The phrase dhikr Allah سبحانه و تعالىincludes everything by means of which a person may draw close to Allah سبحانه و تعالى, such as belief (‘aqeedah), thoughts, actions of the heart, physical actions, praising Allah سبحانه و تعالى, learning and teaching beneficial knowledge, and so on. All of that is remembrance of Allah سبحانه و تعالى. Al-Riyaadh al-Nadrah, p. 245 
The word dhikr means remembrance
In the Islamic context, it is used in the sense of remembrance of Allah سبحانه و تعالى.
The seven heavens and the earth and all that is in them glorify Him (Allah سبحانه و تعالى) and there is not a thing but declares his praise. But you do not understand their glorification. He is truly ever forbearing, oft forgiving. Qur’an.Surah Al-Isara 17:44.
"Read what is revealed to you in the Book and establish regular prayer. Verily, the prayer stops one from indecent and disapproved acts. But verily, dhikr of Allah سبحانه و تعالىis greater and Allah سبحانه و تعالىknows what you do." Qur’an.Surah Al-Ankaboot 29:45.
So (in return for all that) always remember and make mention of Me (when service to Me is due), that I may remember and make mention of you (when judgment and recompense are due); and give thanks to Me, and do not be ungrateful to Me. Qur’an.Surah Al-Baqarah 2:152.
Allah سبحانه و تعالىsays: “O you who believe remember Allah سبحانه و تعالىwith much remembrance, and glorify His praises in the morning and the afternoon.” Qur’an.Surah Al-Ahzaab 33: 41-42. 
On the authority of Abu Moosaa al-Ash’aree رضي الله عنه, the Messenger of Allah سبحانه و تعالىsaid: “The comparison of the one who remembers Allah سبحانه و تعالىand the one who doesn’t is like that of the living and the dead.” Saheeh Al-Bukhari 6407 NE (Vol. 8: 416)
The dhikr after salaah, reported by Abu Hurayrah رضي الله عنه, from the Messenger of Allah ﷺ: “Whoever glorifies Allah سبحانه و تعالى(says Subhaan Allaah) thirty-three times immediately after each prayer, and praises Allah سبحانه و تعالى(says Al-hamdu Lillaah) thirty-three times, and magnifies Allah سبحانه و تعالى(says Allaahu akbar) thirty-three times, this makes ninety-nine, then to complete one hundred says Laa ilaaha ill-Allaah wahdahu laa shareeka lahu, lahu’l-mulk wa lahu’l-hamd wa huwa ‘ala kulli shay’in qadeer (There is no god except Allah سبحانه و تعالىAlone, with no partner, His is the power and His is the praise, and He is Able to do all things) – his sins will be forgiven even if they are like the foam of the sea.” Sahih Muslim 939
Prophet ﷺsaid: “Do not neglect to say in every prayer, Rabbiy a‘inni ‘ala dhikrika wa shukrika wa husni ‘ibaadatika (My Lord, help me to remember You, give thank to You and worship You properly). An-Nasaa’i (1303), Classed as saheeh by al-Albaani رحمت اللهin Saheeh an-Nasaa’i.
’Abdullaah Ibn Busr رضي الله عنهrelated that a man once told the Prophet ﷺ, “The roads to good are many and I am unable to take all of them, so please tell me something to which I can hold fast, but do not overburden me lest I forget it.” He said, “Make sure that your tongue is moist and supple with the remembrance of Allah سبحانه و تعالى, the Exalted.” At-Tirmidhee 9/314 
Dhikr has two meanings:
2) Reminder 
It is defined by Allah سبحانه و تعالىin the Qur’an: “Indeed, by the Remembrance of Allah do hearts find peace.” Qur’an.surah Ar-Raad 13:28 Those who dedicate themselves to regular remembrance of God find peace and tranquility in their hearts, and a wonderful release from the stresses and strains of day-to-day life.
From the mannerisms of dhikr: It is essential that the one performing dhikr rectifies his heart in that dhikr, along with his tongue. There is no benefit for the one who makes dhikr upon the tongue only, while his heart is not in cooperation, reciprocating what is being uttered by the tongue. It is also essential that the one performing the dhikr knows and understands what he is saying in his supplication and what he is asking for
Allah سبحانه و تعالىsays: “and remember your lord with your tongue and within yourself, humbly and with fear and without loud words, in the morning and afternoon and be not of those who are neglectful.”Qur’an.Surah Al-A’araaf: 205.