hello dear friends. thanks for last help. now I have a new problem with meaning of above word in this sentence:
These longitudinal charge density fluctuations are referred to as plasma oscillations

to refer [to]
اشاره کردن [به]

as
به عنوان

These longitudinal charge density fluctuations are referred to as plasma oscillations.
این نوسانات چگالی بار طولی به عنوان نوسانات پلاسما نامیده می شود.

فقط برای بهتر فهمیدن
to as

این نوسانات چگالی بار طولی [به عنوان] نوسانات پلاسما [به] آنها اشاره شده.