run on front end only

my_pmpro_text_translation.php

<?php   // do not copy this line.
/**
 * This recipe changes term "membership" to "subscription" for plugin generated text Front End only
 * 
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_pmpro_gettext_membership($translated_text, $text, $domain)
{
	// Only run on front end, not in admin or AJAX.
	if ( is_admin() || wp_doing_ajax() ) {
		return $translated_text;
	}

	if ( $domain === "paid-memberships-pro" ) {
		$translated_text = str_replace("Membership", "Subscription", $translated_text);
		$translated_text = str_replace("membership", "subscription", $translated_text);
	}
	
	return $translated_text;
}
add_filter("gettext", "my_pmpro_gettext_membership", 10, 3);