Strictly better is a term used to compare cards that are identical in most regards, and in each way they are different the same one is more favorable.  Strictly better does not care about creature type.  For example, Lightning Bolt is strictly better than Shock. Both are instants that cost and deal damage to a creature or player, but Lightning Bolt deals 3 damage where shock deals 2 damage. The opposite of strictly better is called strictly worse, as in "Shock is strictly worse than Lightning Bolt."
The convention is well understood among experienced Magic players. However, those new to the terminology may complain that a strictly better card is not better in all situations than a strictly worse card.   For example, Shock is a better card to draw than Lightning Bolt if both players are at two life and the opponent controls a Booby Trap naming Lightning Bolt. Such examples are not a failure of the terminology; it compares only the attributes of the cards regardless of obscure situations that may arise in play.
Cards are constantly being obsoleted by strictly better versions (sometimes even in the same set, such as Glory Seeker and Knight of Cliffhaven). This may lead to power creep. Examples of strict comparisons include:
More powerful at same cost
The following cards have the same cost but one has an added ability or greater size.
- Alloy Myr is strictly better than Opaline Unicorn because it has greater power.
- Armored Griffin is strictly better than Abbey Griffin because it has greater toughness.
- Ashcoat Bear is strictly better than Grizzly Bears (and its functional reprints) because it also has flash.
- Deviant Glee is strictly better than Unholy Strength as it also can grant activateable trample.
- Elite Vanguard is strictly better than Eager Cadet because it has a greater power.
- Ethereal Armor is strictly better than Lance because it also grants a minimum of +1/+1.
- Giant Crab is strictly better than Homarid Warrior as it doesn't tap to use its ability.
- Hovermyr is strictly better than Omega Myr because it has the added ability of flying and vigilance.
- Makindi Griffin is strictly better than Spotted Griffin because it has greater toughness.
- Sejiri Refuge is strictly better than Coastal Tower because it gains life.
- Sporecap Spider is strictly better than Rib Cage Spider because it has greater toughness.
- Wall of Vines is strictly better than Wall of Wood because it also has reach.
- Yavimaya Wurm is strictly better than Craw Wurm because it has trample.
- Lagonna-Band Trailblazer is strictly better than Yoked Ox because it has an added "Heroic" ability.
- Elvish Mystic is strictly better than Boreal Druid because it creates a green mana that can be spent as colorless or green.
More flexibility at same cost
The following cards have the same effect, but one is more restricted in its use.
- Coordinated Assault is strictly better than Kindled Fury, because it can target an additional creature.
- Fury Charm is strictly better than Shatter, because it has two other selectable modes.
- Jungle Shrine is strictly better than Shivan Oasis, because it can provide an additional color of mana.
- Negate is strictly better than Flash Counter, because it can counter additional types of spells.
- Mother of Runes is strictly better than Benevolent Bodyguard because her ability can be used more than once.
Same effect but cost less
The following cards have the same effect but one has a greater cost.
- Control Magic is strictly better than Mind Control.
- Dead Weight is strictly better than Enfeeblement and Weakness.
- Elvish Archers is strictly better than Hornet Cobra.
- Goblin Tunneler is strictly better than Dwarven Nomad.
- Lightning Bolt is strictly better than Shard Volley.
Instant instead of sorcery
The following are cards that have the same effect and cost but are instants instead of sorceries.
- Boil is strictly better than Boiling Seas.
- Howl from Beyond is strictly better than Bloodcurdling Scream.
Some cards can be compared strictly in more than one way. If one card is better than another in every way that they're different, then the comparison is strict.
- Back to Nature is strictly better than Tranquility because the former costs less and is an instant instead of a sorcery.
- Vulshok Battlegear is strictly better than Warlord's Axe because the former costs less to equip and gives a larger toughness bonus.
- Nova Cleric is strictly better than Squire because the former costs less and has an additional ability.
- Flametongue Kavu is strictly better than Goblin Commando because the latter has less power, costs more, and does less damage when it enters the battlefield.
- Whitesun's Passage is strictly better than Sacred Nectar because the former gains more life and is an instant instead of a sorcery.
- Reprisal is strictly better than Smite the Monstrous because the former prevents regeneration and has a lower mana cost.
- Mark Rosewater. (March 31, 2003.) "This Land is My Land", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Mark Rosewater. (July 20, 2014.) "Does the term "strictly better" care about creature types?", Blogatog, Tumblr.
- Mike Flores. (March 03, 2014.) "Strictly Superior", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Mike Flores. (March 10, 2014.) "Redundancy", Daily MTG, magicthegathering.com, Wizards of the Coast.