The Meaning of "Salaah" in the Language
Shaykh Saalih Ibn Ghaanim as-Sadlaan رحمه الله said, 

"Salaah (صلاة) is the singular for the obligatory Salawaat. It is a noun that is placed in the position of the infinitive. One says: 'I prayed a salaah,' he does not say, 'I prayed tasliyyah.' Salaah from Allaah سبحانه و تعالى means His Mercy and from the servant it means supplication and seeking forgiveness. [1] [2] 

The Meaning of Salaah in the Terminology of the Sharee'ah

It refers to an action consisting of specific pillars and known adhkaar (specific words) that have to meet specific conditions and is performed at set times. Or, it refers to a set of sayings and actions that are commenced by the opening takbeer and completed with the tasleem along with the presence of the intention. [3] 

The Meaning of al-Jamaa'ah in the Language
In the language, al-Jamaa'ah (الجماعة) is derived from jam'. Jam' means to gather together what was dispersed or to join something by bringing together its various parts. It is said, 'I gathered in and it gathered (Jama'tuhu fajtama'a).' Al-Jamaa'ah refers to a number of people who have gathered together for a single purpose. It is also used to to refer to things other than people, so, for example, it is said, 'The jamaa'ah (group or set) of trees and the jamaa' of plants.' With this meaning the word can be applied to a collection of anything. [4] 

The Meaning of al-Jamaa'ah in the Terminology of the Sharee'ah
The meaning of al-Jamaa'ah in the terminology of the jurists is applied to a number of people... Al-Kaasaanee رحمه الله said, 'Al-Jamaa'ah is derived from the meaning of al-ijtimaa' and the least number that can be considered a jamaa'ah is two - an Imaam and a follower.' [5] 

The Meaning of Salaatul Jamaa'ah
[Salaatul Jamaa'ah refers to] the prayer of the follower being linked to and following the prayer of the Imaam while meeting specific conditions. When there occurs a command to perform salaah or a ruling linked to it in the Sharee'ah, it is understood to refer to the legislated Salaah." [6] [7] 

Notes: 

[1] See also: Meaning of the Salah of Allah Upon the Prophet Muhammad (peace be upon him) 
- https://www.muftisays.com/blog/Seifeddine-M/3369_13-07-2013/meaning-of-the-salah-of-allah-upon-the-prophet-muhammad-peace-be-upon-him.html 

[2] Basaa'ir Dhawee at-Tamyeez fee Lataa'if al-Kitaab al-'Azeez (3/434). 

[3] al-Qaamoos al-Fiqhee Lughtana wa Istilaahan (p. 216) of Sa'dee Abu Jaib with slight alteration. 

[4] al-Mawsoo'ah al-Fiqhiyyah (15/280). 

[5] Badaa'i as-Sanaa'i fee Tarteeb ash-Sharaa'i (1/156) of al-Kaasaanee. 

[6] al-Yaaqoot an-Nafees fee Madh-hab ibn Idrees ash-Shaafi'ee (p. 45) by Sayyid Muhammad ash-Shaatiree. 

[7] Shaykh Saalih Ibn Ghaanim as-Sadlaan رحمه الله, Congregational Prayer (Al-Basheer Publications & Translations), pp. 15-16.

See also: The Importance And Virtues Of Salah (Prayer) - http://www.muftisays.com/forums/12-virtues/5039-the-importance-and-virtues-of-salah-prayer.html