.equ (also =)

label .equ value [, page]

This assigns a label with a particular value, so you can then use the label in expressions rather than the constant each time. For example:

do_something = $5421, 3

size_of_array .equ 30
and_factor = %00001111

    ; ...

    ; Select page:

    ld a,:do_something
    out (page_sel_port),a

    ; Do something

    ld b, size_of_array
_loop:
    ld a, (hl)
    call do_something
    and and_factor
    ld (hl), a
    inc hl
    djnz _loop

There is a slight difference between the using = and .equ. = allows you to redefine a label; .equ doesn't.

_label

_label    = 0 ; Works
_label .equ 0 ; Displays an error