The NMAT By GMAC is the primary aptitude test for admission to SVKM's NMIMS - for its flagship Mumbai campus as well as all its other campuses and a host of other institutes. Among the major entrance tests conducted annually, NMAT is the only test that allows a student upto three attempts, and treats the best-of-three score for admissions. 
It is conducted by the Graduate Management Admission Council (GMAC), which also administers the GMAT. 
Some of the prominent institutes (apart from NMIMS) accepting NMAT scores are: 
ISB Hyderabad - Advanced Program in Business Analytics
S P Jain - Mumbai
XIM - Bhubaneshwar

Also, from 2020, NMAT has become an "Adaptive" test, which is very different from a standard MCQ based CBT; as it evaluates your performance on each question and accordingly calibrates the subsequent question(s) as well as the marks you will get for them. Your marks per section can range from 12 to 120 based on your performance.

The candidate should be a graduate with 50% marks or equivalent CGPA in the first attempt. 
Candidates appearing for the final year of Bachelors degree/equivalent qualification examination; and those who have completed their degree requirements but are awaiting results can also apply subject to clearing the final examination in the first attempt with 50% aggregate marks. Any admission given to such candidates is purely provisional, and is confirmed only for successful submission of all valid documents. 
For all the full-time programs, candidates with two or more years of post-qualification work experience may be preferred.

NMAT is typically conducted over a 75-day window from first week of October to mid-December.
However, NMAT 2022 will be held from 10 Oct 2022 to 19 Dec 2022. The dates for each stage of the NMAT process are as under: 

Exam slots will be allotted on a first-come-first-serve basis only. The exam will be available at test centres and exam slots in the Online Proctored format at home on certain dates within the overall testing window, but only with limited capacity.

To apply, the student needs to create an account on nmat.org.in using a valid email address. Using this, the student has to register, enter all his details, upload a photograph and pay the test fee. Post registration, the option to schedule the test is enabled for the user. 
The test fees (excluding taxes) for various stages in the process are as under: 

Exam registration and the application to schools accepting NMAT scores are two different processes. You need to separately apply to each institute that you wish to apply to.