Concerned assertation - "Dude, the cops are coming! splitter that chronic out of the window!"
Noun - "Look at all that splitter."
The criteria for referring to someone as a splitter can vary based on the person asked, the applicant's goal law school(s), how competitive the admissions cycle is, and a number of other factors. One rule of thumb, however, is that a splitter will have one number above the 75th percentile of his/her target school, while the other can be below the 25th percentile (to an extent).
The status of being a splitter is a matter of great concern to law school applicants, and a subject of extensive discussion every admissions cycle. However, some individuals, especially after being admitted, will wear the term as a badge of honor.
Jaime has a 4.0 GPA and a 162 LSAT; he's a reverse splitter.
Peter has a 3.7 GPA and a 172 LSAT; he's not a splitter.
Fred has a 2.6 GPA and a 164 LSAT; he's not a splitter.
There is no such thing as a "reverse splitter", or someone who has a high GPA and a low LSAT.