Definitions:

1. a particular kind or species of matter with uniform properties noun
   • shigella is one of the most toxic substances known to man
2. material of a particular kind or constitution noun
   • the immune response recognizes invading substances
3. the real physical matter of which a person or thing consists noun
   • DNA is the substance of our genes

Synonyms:

• content
• core
• essence
• gist
• heart
• heart and soul
• inwardness
• kernel
• marrow
• meaning
• means
• meat
• message
• nitty-gritty
• nub
• pith
• subject matter
• sum