Students with learning disabilities (LD) make up the largest group of students with of special needs. The are many different definitions for a learning disability as they present differently depending on the child (academically, behaviorally, or social-emotionally). Different professionals (psychology, medicine, education, etc..) will define it differently. However, the Individuals with Disabilities Education Act states: "Specific learning disability means a disorder in one or more of the basic psychological processes involved in understanding or in using language, spoken or written, that may manifest itself in the imperfect ability to listen, think, speak, read, write, spell, or to do mathematical calculations, including conditions such as perceptual disabilities, brain injury, minimal brain dysfunction, dyslexia, and developmental aphasia. Disorders not included. Specific learning disability does not include learning problems that are primarily the result of visual, hearing, or motor disabilities, of intellectual disability, of emotional disturbance, or of environmental, cultural, or economic disadvantage." (https://sites.ed.gov/idea/regs/b/a/300.8/c/10)
In order to identify a child with a learning disability schools use response to intervention or RTI in order to provide early intervention to students. If a student still shown underachievement with proper instruction (RTI) a team should decide what the next step is for that student.
This was a lot of information, if you are concerned about a learning disability talk to the school and try to set out a plan for your child. You are their biggest advocate.