An equilateral triathlon is a triathlon

Triathlon

A triathlon is a multi-sport event involving the completion of three continuous and sequential endurance events. While many variations of the sport exist, triathlon, in its most popular form, involves swimming, cycling, and running in immediate succession over various distances...

 in which each leg would take an approximately equal time. These triathlons were proposed by Wainer and De Veaux (1994) to redress the bias in favour of cycling

Cycling

Cycling, also called bicycling or biking, is the use of bicycles for transport, recreation, or for sport. Persons engaged in cycling are cyclists or bicyclists...

 over running

Running

Running is a means of terrestrial locomotion allowing humans and other animals to move rapidly on foot. It is simply defined in athletics terms as a gait in which at regular points during the running cycle both feet are off the ground...

 and particularly over swimming in standard triathlons. Cycling occupies about 78% of the typical race distances, running 19%, and swimming 3%. Based on world-record times for similar distances, cycling would take 54% of race time, running 31%, and swimming 15%.

In Wainer and De Veaux's equilaterial triathlons, cycling should take approximately 33% of race time, running 33%, and swimming 33% for a ratio of roughly 1:8:3.5 for distances of swimming / cycling / running. Examples would be:

Name	Time per leg	Swim	Bike	Run
Sprint	10 minutes	1 km (0.6 mi)	8.5 km (5.3 mi)	3.9 km (2.4 mi)
Olympic	28 minutes	2.7 km (1.7 mi)	22.4 km (13.9 mi)	10 km (6.2 mi)
Ironman	127 minutes	12 km (7.5 mi)	96.2 km (59.8 mi)	42.2 km (26.2 mi)

Wainer and De Veaux's proposal appears to have had little influence on triathlon race distances.