Decline is a verb meaning to refuse politely, or to become smaller, fewer, or less; decrease.
In business context, decline often refers to a decrease in sales, profits, or market share.
The term "decline" also has other meanings:
1. To slope downward (geography)
2. To approach the end of a period of time (e.g. "the day was declining")
3. In grammar, to state the forms of a noun, pronoun, or adjective in a language