Any place of prostration

     "Ju'ilat liy al-ardha masjidan wa tahooran."
     "The earth has been made for me a masjid (lit: place of prostration) and pure."  Muslim & Bukhari

     {Qul amara rabbiy bil qisti wa aqeemoo wujoohakum 'inda kulli masjidin wad'oohu mukhliseena lahu ad-deena kama
     bada'akum ta'udoon.}
     {Say, my Lord has commanded justice and that you direct your faces at every masjid and call Him sincerely.  The
     deen belongs to Him.  Just as He started you out, likewise you will return.}  Al-A'raaf:29

Structures dedicated to this purpose.  This is the most common meaning of the word.  They are places dedicated to the worship of Allah, to calling mankind to guidance, informing them of the deen of truth, meeting the needs of the Muslims and organizing their affairs.  It is a place for Muslims to be away from the distractions of everyday life, earning a living, etc. for some moments of worship and brotherhood.  It is the foundation of the Muslim community and the Muslim nation.  "Worship" should not be understood in the narrow sense in which Christians and others use the word.  When the Prophet (sas) wanted to convene a war council or take consultation and/or make preparations for an offensive or defensive military move, it was done in the masjid.

The masjid is a place where Muslim meets Muslim before Allah Most High.  There is no difference between the friend or the stranger, the relative or the unrelated, the wealthy or the poor, the powerful or the weak.  All come before Allah as slaves in obedience and worship of the One.

Some Etiquettes of the Masjid

   1.If you hear the adhan and you are adult, male, sane, not ill and not taking a meal, you must proceed to the masjid and make salat with
     the jama'a.  If you do not, then you will have missed that prayer altogether and there is no way you can "make it up".  The Prophet
     (sas) said:

     "Man sami'a an-nidaa'a fa lam ya'tihi fa laa salaata lahu."
     "Whoever hears the call and does not come to it has no prayer."  (Abu Daud - hadith sahih)

   2.Enter the masjid with your right foot first and say:

     "Bismillahi.  As-salaatu wa as-salaamu 'alaa rasooli Allahi.  Allahumma ighfir liy dhunoobiy waftah liy abwaaba rahmatika"
     "In the name of Allah.  Prayers and peace of Allah be upon Allah's Messenger.  O Allah!  Forgive my sins and open for me
     the doors of your mercy."

   3.Do not sit until you have prayed at least two raka'aat.  If there is some specific sunnah or nafl prayer to be made, that is sufficient.
     Otherwise, pray two raka'aat as "tahiyyatu al-masjid" (greeting the masjid).

   4.Do not pass in front of anyone who is praying unless they have placed a sutra in front of themselves.  You may pass in front of the
     sutra.  Try to avoid passing in front of someone at any distance and definitely avoid it within three arms lengths.

   5.Seek to be in the farthest row possible toward the front.  This is where the most blessing is.  Do not seek out the back or side wall for
     "comfort" and neglect this opportunity for increased blessing.

   6.Do not disturb those already present in order to get to the front rows.

   7.Do not be bothersome nor cause any discomfort to those next to you in the line.

   8.Place your feet at shoulder's width apart.

   9.Be in contact with those next to you with your feet and your shoulders.

  10.Be in remembrance and mentioning of Allah as much as possible the whole time you are in the masjid.

  11.Leave with your left foot first and say:

     "Bismillahi.  As-salaatu wa as-salaamu 'alaa rasooli Allahi.  Allahuma ighfir liy dhunoobiy waftah liy abwaaba fadhlika."
     "In the name of Allah.  Prayers and peace of Allah be upon Allah's Messenger.  O Allah!  Forgive my sins and open for me
     the doors of your plenty."

The masjid is for those who truly believe in Allah and in the Last Day, who establish their prayer faithfully, pay their zakat and fulfill all
obligations of Islam and who fear no one but Allah.  These characteristics are the only way to guidance and the people of these
characteristics are the ONLY ones who rightfully erect, occupy, govern and control the masajid in Islam.  Allah said:

     {Innamaa ya'muru masaajida Allahi man aamana billahi wa al-yaumi al-aakhiri wa aqaama as-salaata wa
     aataa az-zakaata wa lam yakhsha illaa Allaha fa 'asaa ulaa'ika an yakoona min al-muhtadeen.}
     {The only ones to occupy Allah's mosques are whoever believes in Allah and in the Last Day, establishes the prayer,
     pays the zakat and fears none but Allah.  Perhaps these will be among the guided ones.}  At-Tauba:18

So, the masjid in its formal, terminological or "narrow" meaning is all buildings raised by Muslims for the purposes described earlier:
worship of Allah, remembrance of Allah and organizing the affairs of the Muslims.  Allah said:

     {Fiy buyootin adhina Allahu an turfa'a wa yudhkara feehaa ismuhu yusabbihu lahu feehaa bi al-ghuduwwi wa
     al-aasaali.  Rijaalun laa tulheehim tijaaratun wa laa bei'un 'an dhikri Allahi wa iqaami as-salaati wa eetaa'i
     az-zakaati yakhaafoona yauman tataqallabu feehi al-quloobu wa al-absaaru.}
     {In houses that Allah allowed to be raised and in which His name is mentioned.  He is exalted therein morning and
     afternoon by men who are not distracted by business and selling from the remembrance of Allah, establishing the
     prayer and paying the zakat.  They fear a day on which hearts and sight will be overturned.}  An-Nur:36-37

Al-hamdu lillahi, Allah has made it easy for us to establish our masjid.  Now it is up to us to live up to being ahlun (the
appropriate people) for this beautiful house of Allah.  When 'Uthman built the Prophet's masjid in Madina (or re-built it), that
people had this and that to say about it.  'Uthman said:

     Innakum akthartum 'alayya wa inniy sami'tu rasoola Allahi yaqoolu:  "Man banaa masjidan yabtaghiy bihi wajha
     Alahi banaa Allahu lahu baitan fiy al-jannati."  Wa fiy riwaayatin:  "...banaa Allahu lahu mithlahu fiy al-jannati."
     "You have become many to me and I heard Allah's Messenger say:  'Whoever builds a masjid seeking therewith the
     countenance of Allah, Allah will build him a house in Paradise.'  In another narration: '...Allah will build him a
     house equal to it in Paradise." Muslim & Al-Bukhari

Although the masjid is not just for "worship" in the narrow sense of the word and it is also to be used for organizing and
facilitating the affairs of the Muslim community, it is not to be used for stictly worldly or personal pursuits.  So much so, that the
Prophet (sas) has instructed us to make a negative du'a for one who crosses that line.  The Prophet (sas) said:

     "Man sami'a rajulan yunshidu dhaalatan fiy al-masjid fal yaqul:  Laa raddahaa Allahu 'alaika, fa inna al-masjida lam
     tubna li hadhaa."
     "Whoever hears a man asking about a lost animal in the masjid, let him say to him: "May Allah not return it to
     you.  The masjid was not built for this.'"  Muslim