Sometimes we may want to change the wording of the language used in a plugin or theme, but don't want to edit code directly. A cleaner way to go is to use the built in filter that WordPress has called 'gettext'. This filter will search your codebase for translatable strings and replace when an exact match is found.

a-gettext-filter-4-pmpro.php

<?php
/**
 * This filter will search your codebase for translatable strings and replace when an exact match is found.
 * 
 * Here we're changing 'Membership' to 'Subscription' for Paid Memberships Pro.
 * 
 * Add this code to your PMPro Customizations Plugin
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 * Note: When adding to your Customizations Plugin, be careful not to include the opening php tag on line 1 above.
 *
 * @param  string $output_text     this represents the end result
 * @param  string $input_text      what is written in the code that we want to change
 * @param  string $domain          text-domain of the plugin/theme that contains the code
 *
 * @return string                  the result of the text transformation
 */
function my_gettext_membership( $output_text, $input_text, $domain ) {
	if ( ! is_admin() && 'paid-memberships-pro' === $domain ) {
		$output_text = str_replace( 'Membership Level', 'Subscription', $output_text );
		$output_text = str_replace( 'membership level', 'subscription', $output_text );
		$output_text = str_replace( 'membership', 'subscription', $output_text );
		$output_text = str_replace( 'Membership', 'Subscription', $output_text );
	}
	return $output_text;
}
add_filter( 'gettext', 'my_gettext_membership', 10, 3 );

Newer version that supports _n calls here: https://gist.github.com/ideadude/ab26eb858920b7bcb546d658e6c92178